Nitrous - PsychonautWiki

Nitrous

(Redirected from Nitrous Oxide)
Summary sheet: Nitrous
Nitrous
Nitrous Oxide.svg
Chemical Nomenclature
Common names Nitrous Oxide, Laughing Gas, Nitrous, Hippy Crack, NOS, Nitro, N2O, Nangs
Systematic name Dinitrogen monoxide
Class Membership
Psychoactive class Dissociative
Chemical class Nitrogen oxide
Routes of Administration

WARNING: Always start with lower doses due to differences between individual body weight, tolerance, metabolism, and personal sensitivity. See responsible use section.

Inhaled
Dosage
Threshold 4 g
Light 4 - 8 g (1/2 - 1 cartridges)
Common 8 - 16 g (1 - 2 cartridges)
Strong 16 - 40 g (2 - 5 cartridges)
Heavy 40 g + (>5 cartridges)
Duration
Total 1 - 5 minutes
Onset 5 - 10 seconds
Come up 5 - 10 seconds
Peak 15 - 30 seconds
Offset 1 - 5 minutes
After effects 15 - 30 minutes











DISCLAIMER: PW's dosage information is gathered from users and resources for educational purposes only. It is not a recommendation and should be verified with other sources for accuracy.

Interactions
Alcohol
GHB
GBL
Opioids
Tramadol

Skull and crossbones darktextred2.png

Nitrous oxide can cause irreversible neurotoxic damages

Frequent and/or chronic use can cause vitamin B12 depletion, such as other rapidly vitamin B12 depleting substance, may lead to severe nerve damage.[1] We recommend that you always supplement with vitmamin B12 if you use this substance, regardless dose and frequency to be on the safe side. Especially those with "decreased vitamin B12 absorption, such as in those with small bowel resection, irritable bowel disease, and/or pernicious anemia, and those with reduced intake, such as in vegans and vegetarians".[2] Additionally, improper use puts the user at risk of oxygen deprivation. It is highly advised to use harm reduction practices if using this substance.


Nitrous oxide (also known as laughing gas, nitrous, NOS, and informally as hippy-crack). Whipped cream chargers/cartridges are called whippits[3] (also spelled “whippits”), nangs, or bulbs.

Nitrous oxide is an atypical dissociative substance that is found in the form of an inhalable gas. The exact mechanism of action of nitrous oxide is unknown, but it is thought to have an effect on GABA and NMDA receptors in the brain.

Nitrous oxide was first identified in 1772 by the British chemist Joseph Priestly.[4] In the 19th century, it was discovered to have anesthetic properties which made it suitable for dental and surgical procedures.[5] It was given the name "laughing gas" by the British chemist Humphry Davy due to its exhilarating and laughter-inducing effects. Today, it is widely used in surgery and dentistry for its anaesthetic and analgesic effects, and in motorsports for its performance boosting effect on internal combustion engines. Additionally, it has a number of industrial uses and is commonly sold as a whipped cream propellant in kitchen and household stores.

Subjective effects include sedation, pain relief, motor control loss, anxiety suppression, conceptual thinking, euphoria, and dissociation.

Nitrous is considered to have low to moderate abuse potential and low toxicity when used in moderation.

Whipped-cream chargers are often combined with otherwise empty whip cream dispensers as a convenient route of administration.

History and culture

Nitrous oxide gas was first synthesized in 1772 by English natural philosopher and chemist Joseph Priestley.[6] He published his findings in the book Experiments and Observations on Different Kinds of Air (1775), describing the synthesis of the gas by heating iron filings dampened with nitric acid.[7] In 1800, Humphry Davy, an assistant at the Pneumatic Institute, published a book about nitrous oxide in 1800.[8] In the book, there is the following passage;

"As nitrous oxide in its extensive operation appears capable of destroying physical pain, it may probably be used with advantage during surgical operations where too great an effusion of blood does not take place."

Davy notes the analgesic effect of nitrous oxide and its potential for use in surgical operations.[8] Davy coined the name "laughing gas" for nitrous oxide.[9] Beginning in 1799, nitrous oxide was used as a recreational drug at "laughing gas parties". They became an immediate success within the British upper class. Despite Davy's discovery of nitrous' potential for anesthesia, doctors did not attempt to use it for nearly half a decade.[10]

The discovery of ether in 1830 led nitrous to fall out of popularity as a recreational drug. It was not until 1844 that American dentist Horace Wells noticed the anesthetic properties of the gas.[8] With the assistance of Gardner Quincy Colton and John Mankey Riggs, he demonstrated insensitivity to pain during a tooth extraction.[5] The practice was not immediately adopted by other dentists, as Wells' first public demonstration had been partly unsuccessful. [11] In 1863, Gardner Quincy Colton began to administer the gas to patients in all of his "Colton Dental Association" clinics, administering nitrous oxide to over 25,000 patients over three years. This brought the practice into general use.[12]

Nitrous oxide was not found to be a strong enough anesthetic for major surgeries in hospital settings, but it became useful as an initiator for stronger anesthetics like ether or chloroform.[13] Hospitals would initiate treatment with a mild flow of nitrous oxide, and then gradually increase the flow of the stronger anesthetic.[12] This technique of initiating anesthesia is still used in hospitals today.

Nitrous is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.[14]

Chemistry

Nitrous oxide, or dinitrogen monoxide, was first synthesised by heating ammonium nitrate in the presence of iron filings and then passing the gas that came off (NO) through water.[15] Now it is commonly synthesised by gently heating ammonium nitrate to decompose it into nitrous oxide. It is an oxide of nitrogen.

Nitrous has a linear molecular structure. It can be thought as existing in two "resonance structures":

  • The central nitrogen atom is triple bonded to the other nitrogen atom and single bonded to the oxygen atom.
  • The central nitrogen atom double-bonded to both the other nitrogen atom as well as the oxygen atom.

In practice, the atom will almost exclusively exist in between these two states almost all the time, and the electrons will be delocalised across the molecule. Each of the resonance forms can be thought of as having a positive and negative charge, giving a neutral molecule overall.

Nitrous is used as an oxidiser in rocketry and in motor racing to increase the power output of engines. At elevated temperatures, nitrous oxide is a powerful oxidizer similar to molecular oxygen.[citation needed]

At room temperature, nitrous is a colorless and non-flammable gas with a slightly sweet odor and distinctive sweet taste.

Pharmacology

Further information: NMDA receptor antagonist

Although N2O affects quite a few receptors, its anesthetic, hallucinogenic, and euphoriant effects are likely caused predominantly or fully via its effects as an NMDA receptor antagonist.[16][17] NMDA receptors allow for electrical signals to pass between neurons in the brain and spinal column; for the signals to pass, the receptor must be open. Dissociatives close the NMDA receptors by blocking them. This disconnection of neurons leads to loss of feeling, difficulty moving, and eventually the famous “hole”.

The pharmacological mechanism of action behind N2O in medicine is not entirely known. However, it has been shown to directly modulate a broad range of receptors and this likely plays a significant role in many of its effects. It moderately blocks β2-subunit-containing nACh channels, weakly inhibits AMPA, kainate, GABAA-rho, and 5-HT3 receptors and slightly potentiates GABAA and glycine receptors.[16][18] It has also been shown to activate two-pore-domain K+ channels.[19]

Subjective effects

Disclaimer: The effects listed below cite the Subjective Effect Index (SEI), an open research literature based on anecdotal user reports and the personal analyses of PsychonautWiki contributors. As a result, they should be viewed with a healthy degree of skepticism.

It is also worth noting that these effects will not necessarily occur in a predictable or reliable manner, although higher doses are more liable to induce the full spectrum of effects. Likewise, adverse effects become increasingly likely with higher doses and may include addiction, severe injury, or death ☠.

Physical effects
 

Visual effects
 

Cognitive effects
 

Auditory effects
 

Multi-sensory effects
 

Transpersonal effects
 

Combinational effects

Nitrous is commonly combined with other hallucinogens for it which it acts as a brief but profound potentiator of their effects.

  • Psychedelics - When taken in combination with a psychedelic such as LSD or psilocybin mushrooms, the effect will be a sudden and dramatic increase in perceived geometry to its maximum level of 8A or 8B. This is alongside a sudden and dramatic ego death.
  • Dissociatives - When taken in combination with a dissociative such as MXE or DXM, the effect will be a sudden and dramatic increase in disconnective effects and the triggering of a sudden internal hallucinatory scenario.
  • Cannabis - When taken in combination with cannabis, the overall effects of the nitrous itself are potentiated more so than the cannabis' effects.
  • Alcohol - When taken in combination with alcohol, adverse side effects such as confusion, dizziness, and headaches are often significantly increased.

Experience reports

Anecdotal reports which describe the effects of this compound within our experience index include:

Additional experience reports can be found here:

Available forms

Medical nitrous oxide

Medical tanks - These are hard to find and dangerous without a professional. They are occasionally seen at music festivals being used to fill balloons for sale.

Nonmedical nitrous oxide

 

Please recycle empty whipped cream containers.

 
Whipped-cream chargers dumped in nature.

 
The canister on the left is whipped cream, a product which is pressurized with nitrous oxide. The two canisters on the right contain 'flavoured' oxygen.
 
Nitrous oxide tubes.
  • Tubes - These are now commonly sold by web shops that are specializing in nitrous oxide.
  • Canned whipped cream - These are found in any grocery store. They contain very minimal gas with one or two uses before the cream comes out.
  • Whipped-cream charger - These are readily available and cheap to purchase online. They are small metal canisters that can be used by a nitrous cracker to fill a balloon full of gas which is then inhaled. Some varieties contain industrial residue and strength vary (as it is food grade).

Toxicity and harm potential

The exact toxic dosage is unknown. Potential problems include:

  • Nitrous temporarily inhibits methionine synthase, a B12-dependant enzyme responsible for many important functions in the body.[21] Heavy and frequent long-term use can deplete vitamin B12 in the body and lead to serious and unpleasant neurological problems. Users may experience numbness and tingling in the fingers, toes, lips, et al. In more severe cases, there will be numbness of all extremities.[22] Taking B12 supplements, especially in combination with a multivitamin and complete amino acid supplements, may help alleviate this problem. If one experiences these symptoms, nitrous use should be ceased immediately, and if the symptoms persist, medical attention should be sought after.
  • Nitric oxide, a toxic industrial gas, is occasionally mistaken for nitrous oxide. Users should be careful they know what they are inhaling. Inhaling Nitric Oxide can permanently damage the lungs or kill.

It is strongly recommended that one use harm reduction practices when using this substance.

Risks of whipped-cream chargers compared to medical nitrous oxide

Medical nitrous oxide has been safely used as a mild anesthetic for over 150 years.

  • Vapor pressure
    • Whipped-cream chargers: Very cold temperatures of the gas can freeze the lips and throat if taken directly from a tank or whippet. Releasing the gas into a balloon first allows the gas to warm before being administered.
    • Medical nitrous oxide is released slowly.
  • Hypoxia (abnormally low level of oxygen in the tissues), particularly the entity 'diffusion hypoxia'.[23]
    • Whipped-cream chargers contains no oxygen. Oxygen deprivation that can cause brain injury and suffocation can result from lack of oxygen from pure nitrous oxide continuously inhaled from strap-on mask connected to a gas canister. Never use nitrous in any manner that does not provide for adequate oxygen intake. Suffocation can occur without discomfort.[24]
    • Medical grade nitrous oxide contains 50% nitrous oxide, and 50% oxygen. When used as an anesthetic, nitrous is always administered in combination with oxygen.
  • Purity
    • Whipped-cream chargers contains food grade. Thus, they can contain a range of impurities, such as industrial grease left over from manufacturing, and tiny particles of steel from the pierced metal. Users can filter them through a piece of cotton, cloth, or clothing to reduce the amount of impurities inhaled.[25][26] Regular cleaning of nitrous dispensing devices can also reduce the harm posed by impurities.
    • Medical grade nitrous oxide: Contains medical grade nitrous oxide.

Dependence and abuse potential

As with other dissociatives, the chronic use of nitrous oxide can be considered mildly addictive with a moderate potential for abuse.

Tolerance to many of the effects of nitrous oxide develops with prolonged and repeated use. This results in users having to administer increasingly large doses to achieve the same effects. After that, it takes about 3 - 7 days for the tolerance to be reduced to half and 1 - 2 weeks to be back at baseline (in the absence of further consumption). Nitrous oxide does not produce cross-tolerance with other dissocatives, meaning that after the use of nitrous oxide other dissociatives will not have a reduced effect.

Dangerous interactions

Warning: Many psychoactive substances that are reasonably safe to use on their own can suddenly become dangerous and even life-threatening when combined with certain other substances. The following list provides some known dangerous interactions (although it is not guaranteed to include all of them).

Always conduct independent research (e.g. Google, DuckDuckGo, PubMed) to ensure that a combination of two or more substances is safe to consume. Some of the listed interactions have been sourced from TripSit.

  • Alcohol - Both substances potentiate the ataxia and sedation caused by the other and can lead to unexpected loss of consciousness at high doses. While unconscious, vomit aspiration is a risk if not placed in the recovery position. Memory blackouts are likely.
  • GHB - Both substances potentiate the ataxia and sedation caused by the other and can lead to unexpected loss of consciousness at high doses. While unconscious, vomit aspiration is a risk if not placed in the recovery position. Memory blackouts are likely.
  • GBL - Both substances potentiate the ataxia and sedation caused by the other and can lead to unexpected loss of consciousness at high doses. While unconscious, vomit aspiration is a risk if not placed in the recovery position. Memory blackouts are likely.
  • Opioids - Both substances potentiate the ataxia and sedation caused by the other and can lead to unexpected loss of consciousness at high doses. While unconscious, vomit aspiration is a risk if not placed in the recovery position. Memory blackouts are likely.
  • Tramadol - Both substances potentiate the ataxia and sedation caused by the other and can lead to unexpected loss of consciousness at high doses. While unconscious, vomit aspiration is a risk if not placed in the recovery position. Memory blackouts are likely.

Legal status

 

This legality section is a stub.

As such, it may contain incomplete or wrong information. You can help by expanding it.

  • Germany: Nitrous oxide is not a controlled substance under the BtMG (Narcotics Act) or the NpSG (New Psychoactive Substances Act).[27][28] It is legal, as long as it is not sold for human consumption, according to §2 AMG.[29] If it is intended for medical use, it is classified as a prescription medicine.[30]
  • India: Nitrous oxide is available for general anesthesia purposes. India's gas cylinder rules (1985) permit the transfer of gas from one cylinder to another for breathing purposes.[citation needed]
  • Netherlands:
     
    Sign stating "forbidden to use nitrous oxide" in the Poelestraat in Groningen, Netherlands, which is also known as the main nightlife area with bars, pubs, and clubs. Since 1 January 2021 has the use of nitrous oxide in the area been prohibited.
    Since 1 January 2023, the possession, importing, and sale of nitrous oxide is banned in the Netherlands, with exceptions for medical use, and the food industry.[31] It is illegal under the Opium Act.[32]
  • New Zealand: Nitrous oxide is a prescription medicine, and its sale or possession without a prescription is an offense under the Medicines Act.[citation needed]
  • United Kingdom: It is illegal to supply nitrous oxide for its psychoactive effects under the Psychoactive Substances Act 2016[33], However "cream chargers" are sold legally.
  • United States: Possession of nitrous oxide is legal under federal law and is not subject to DEA purview.[34] It is, however, regulated by the Food and Drug Administration under the Food Drug and Cosmetics Act. Prosecution is possible under its "misbranding" clauses, prohibiting the sale or distribution of nitrous oxide for the purpose of human consumption. Many states have laws regulating the possession, sale, and distribution of nitrous oxide. Such laws usually ban distribution to minors or limit the amount of nitrous oxide that may be sold without a special license. For example, in the state of California possession for recreational use is prohibited and qualifies as a misdemeanor.[35]

Typical equipment

Balloons

A regular balloon can typically hold a single 8 gram whipped-cream charger. However, jumbo balloons can hold about 3 cartridges.

Gas filter

Cloth is used to filter the gas from whipped-cream chargers that contains often contains small amounts of impurities.

Gas transfer equipment

A typical whipped cream cartridge contain 8 gram nitrous oxide.

Whipped-cream charger cracker

 
A nitrous charger cracker.
Frostbite from nitrous charger crackers

A nitrous charger cracker repeatedly used to decompress cartridges without an outer layer of silicone can cause frostbite.

Tube valves and regulators

A plastic valve mounted on a tube
A gas pressure regulator mounted on a tube

For nitrous oxide tubes, simple plastic valves, or gas pressure regulators that indicate the gas pressure. These equipments are often sold by web shops that are specialized in nitrous oxide.

Whipping dispensers
 
Nitrous canisters made entirely of metal won't explode.

To use a half-pint or one-pint whipped cream dispenser, you will need one to two chargers, which is equivalent to 8-16 g. For a one-liter dispenser, you will need two to three chargers, which is equivalent to 16-24 g. The same quantity of gas can also be filled from a large tube with an attached gas pressure regulator connected to the whipping dispenser.

It is important not to overcharge your whipped cream dispenser.

Risks
Direct inhalation

Whipping dispensers is more convenient to fill balloons with than crackers. Direct inhalation of butane can cause frostbite like effects that pose a serious risk of personal injury.

Dangers from plastic whip cream dispensers

It is advised to check that all parts of a whipping dispenser are made of metal, because dispensers made of plastic have exploded and damaged, and albeit extremely rare, even killed people:

  • In June 2017, Rebecca Burger, 33, a fitness blogger and model, died after a whipped-cream charger exploded and struck her in the chest.
  • In August 2017, another woman got wounded from a whipping dispenser.[36]

See also

External links

Harm reduction

Discussion

Literature

  • Sheldin, M., Wallechinsky, D., Salyer, S. Laughing Gas: Nitrous Oxide. Ronin Publishing, 1993
  • Humphry Davy. Researches Clinical and Philosophical, Chiefly Concerning Nitrous Oxide, 2010

References

  1. Flippo, T. S. (1 December 1993). "Neurologic Degeneration Associated With Nitrous Oxide Anesthesia in Patients With Vitamin B12 Deficiency". Archives of Surgery. 128 (12): 1391. doi:10.1001/archsurg.1993.01420240099018. ISSN 0004-0010. 
  2. Campdesuner, V; Teklie, Y; Alkayali, T; Pierce, D; George, J (9 July 2020). "Nitrous Oxide-Induced Vitamin B12 Deficiency Resulting in Myelopathy". Cureus. 12 (7): e9088. doi:10.7759/cureus.9088. PMC 7366039 . PMID 32685323. 
  3. Tarendash, A. S. (July 2001). Let’s Review: Chemistry, the Physical Setting. Barrons Educational Series. ISBN 9780764116643. 
  4. Keys, T. E. (1 September 1941). "THE DEVELOPMENT OF ANESTHESIA". Anesthesiology. 2 (5): 552–574. doi:10.1097/00000542-194109000-00008. ISSN 0003-3022. 
  5. 5.0 5.1 Erving, H. W. (May 1933). "The Discoverer of Anæsthesia: Dr. Horace Wells of Hartford". The Yale Journal of Biology and Medicine. 5 (5): 421–430. ISSN 0044-0086. 
  6. Goerig, M., Schulte am Esch, J. (1 September 2001). "History of nitrous oxide—with special reference to its early use in Germany". Best Practice & Research Clinical Anaesthesiology. 15 (3): 313–338. doi:10.1053/bean.2001.0165. ISSN 1521-6896. 
  7. Priestley, J., Johnson, J., Adams, J., John Adams Library (Boston Public Library) BRL (1775). Experiments and observations on different kinds of air / by Joseph Priestly, LL.D.F.R.S. London : Printed for J. Johnson ... 
  8. 8.0 8.1 8.2 Boyle, H. E. G. (27 January 1934). "NITROUS OXIDE: HISTORY AND DEVELOPMENT". BMJ. 1 (3812): 153–155. doi:10.1136/bmj.1.3812.153. ISSN 0959-8138. 
  9. Hardman, J. G., Hopkins, P. M., Struys, M. M. R. F., eds. (2017). Oxford textbook of anaesthesia. Oxford textbooks in anaesthesia. Oxford University Press. ISBN 9780199642045. 
  10. "The Consumers Union on Licit and Illicit Drugs". Science News. 102 (23): 357. 2 December 1972. doi:10.2307/3957382. ISSN 0036-8423. 
  11. Desai, S., Desai, M., Pandav, C. (1 December 2007). "The Discovery of Modern Anaesthesia – Contributions of Davy, Clarke, Long, Wells and Morton". Anesthesiology and Perioperative Medicine Publications. 51 (6). 
  12. 12.0 12.1 Miller, A. H. (1 July 1941). "TECHNICAL DEVELOPMENT OF GAS ANESTHESIA". Anesthesiology. 2 (4): 398–409. doi:10.1097/00000542-194107000-00004. ISSN 0003-3022. 
  13. Sneader, W. (2005). Drug discovery: a history (Part 1: Legacy of the past, chapter 8: systematic medicine). Wiley. ISBN 9780471899792. 
  14. World Health Organization (2017), WHO model list of essential medicines, 20th list (March 2017, amended August 2017), World Health Organization, retrieved 24 August 2017 
  15. Nitrous Oxide 
  16. 16.0 16.1 Yamakura, T., Harris, R. A. (1 October 2000). "Effects of Gaseous Anesthetics Nitrous Oxide and Xenon on Ligand-gated Ion Channels". Anesthesiology. 93 (4): 1095–1101. doi:10.1097/00000542-200010000-00034. ISSN 0003-3022. 
  17. Emmanouil, D. E., Quock, R. M. (2007). "Advances in understanding the actions of nitrous oxide". Anesthesia Progress. 54 (1): 9–18. doi:10.2344/0003-3006(2007)54[9:AIUTAO]2.0.CO;2. ISSN 0003-3006. 
  18. Mennerick, S., Jevtovic-Todorovic, V., Todorovic, S. M., Shen, W., Olney, J. W., Zorumski, C. F. (1 December 1998). "Effect of nitrous oxide on excitatory and inhibitory synaptic transmission in hippocampal cultures". The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 18 (23): 9716–9726. ISSN 0270-6474. 
  19. Gruss, M., Bushell, T. J., Bright, D. P., Lieb, W. R., Mathie, A., Franks, N. P. (February 2004). "Two-pore-domain K+ channels are a novel target for the anesthetic gases xenon, nitrous oxide, and cyclopropane". Molecular Pharmacology. 65 (2): 443–452. doi:10.1124/mol.65.2.443. ISSN 0026-895X. 
  20. Evans EB, Evans MR (November 2021). "Nangs, balloons and crackers: Recreational nitrous oxide neurotoxicity". Aust J Gen Pract (Review). 50 (11): 834–838. doi:10.31128/AJGP-10-20-5668 . PMID 34713284.  Unknown parameter |s2cid= ignored (help)
  21. Weimann, J. (March 2003). "Toxicity of nitrous oxide". Best Practice & Research Clinical Anaesthesiology. 17 (1): 47–61. doi:10.1053/bean.2002.0264. ISSN 1521-6896. 
  22. Ok-Ad5515 (2021), Warning about Nitrous Oxide 
  23. https://pubmed.ncbi.nlm.nih.gov/3537624/
  24. http://www.erowid.org/chemicals/nitrous/nitrous_health.shtml
  25. https://www.dancewizensw.org.au/filtering-nangs
  26. http://www.erowid.org/chemicals/nitrous/nitrous_article2.shtml
  27. BtMG | http://www.gesetze-im-internet.de/btmg_1981/BtMG.pdf
  28. NpSG | https://www.gesetze-im-internet.de/npsg/NpSG.pdf
  29. § 2 AMG - Einzelnorm 
  30. Anlage 1 AMVV | https://www.bfarm.de/SharedDocs/Downloads/DE/Arzneimittel/Pharmakovigilanz/Gremien/Verschreibungspflicht/liste_stoffe_zubereitungen.pdf;jsessionid=B4E9C9724EA80BC179A1B9384A42240D.2_cid354?__blob=publicationFile&v=25
  31. Rankin, Jennifer (2022-11-15). "Dutch to ban laughing gas over fears for health and road safety". The Guardian (in English). ISSN 0261-3077. Retrieved 2023-10-20. 
  32. "Opium act (drugs)". www.belastingdienst.nl (in English). Retrieved 2023-10-20. 
  33. Participation, E., Psychoactive Substances Act 2016 
  34. "US Nitrous Oxide Laws (alphabetically) Based on a search of online free legal databases. Conducted May 2002" - http://www.cognitiveliberty.org/dll/N20_state_laws.htm
  35. California Code, Penal Code - PEN § 381b 
  36. https://www.independent.co.uk/news/world/woman-whipped-cream-canister-explosion-emilie-lada-skull-crack-award-ps990000-french-court-model-death-a7880461.html