For a long time, it has been sufficient for us to believe that ”Lavender works” because other people tell us “it worked for us”; and because it feels natural to believe that something which smells so good and looks so good is likely to be doing some good. It’s a kind of faith.
But it’s not always enough: just for one example, phosgene (used as a poison gas against troops during WW1) smells of pear-drops. I remember grandfather banning those sweets from his house – and from my breath. The other trouble with this way of gathering evidence is that you rarely or never hear from the people killed by stuff, whatever it was.
Nor is it enough to say “Lavender is a natural cure”. Deadly Nightshade is natural too. Though it does have carefully judged medicinal uses, anyone trying an infusion of its essential oil would swiftly tell us the name is accurate – except Deadly Nightshade destroys the power of speech.
All of this adds up to a reason why Professor René-Maurice Gattefossé (1881-1950) became such an important figure in the 20th century history of how and why we believe in the effectiveness of Lavender.
Gattefossé was born into one of France’s most important chemical and perfumery families. It still is – and it’s worth reading part of their website’s  introduction to the Business: Established in 1880, Gattefossé is a…Group which develops, manufactures and markets innovative products and application technologies…[ ]… the Group is an innovative leader in lipidic excipients and novel specialty ingredients. As oleochemistry  specialists
we offer a range of functional lipid excipients.
By 1907 René-Maurice himself was working in a group to investigate and publish scientific studies of aromatic substances. This circumstance has been enough to explain the persistence of a myth that Gattefossé had a whole vat of Lavender oil open on his workbench into which he ‘accidentally’ plunged his arm after burning it – a ludicrous idea, as any chemist would know. What he did do was develop the visionary idea that studying the therapeutic use of essential oils of many plants could constitute an academic, scientific discipline in its own right. Indeed, the very word aromatherapy is one which he brought to writing, in his substantial book on the topic in 1937.
The story about his accident, he himself told in his book, (in translation) 
“The external application of small quantities of essences rapidly stops the spread of gangrenous sores. In my personal experience, after a laboratory explosion covered me with burning substances which I extinguished by rolling on a grassy lawn, both my hands were covered with a rapidly developing gas gangrene. Just one rinse with lavender essence stopped “the gasification of the tissue”. This treatment was followed by profuse sweating, and healing began the next day (July 1910).”
Gas gangrene caused tens of thousands of amputations and deaths during WW1 even among the apparently lightly wounded. In these cases it seems to have been caused by a bacterium prevalent in the dreadfully polluted soils of the war. In Gattefossé’s case it is quite probable that he literally picked it up during his roll in the grass. However his burns became infected, he (or someone else: his statement does not say) applied the Lavender essential oil deliberately, believing it would work and proving that it did.
However, remember that Gattefossé had his accident more than 100 years ago, and medical knowledge has moved on. DO NOT USE HIS METHOD YOURSELF. http://www.nhs.uk/Conditions/Burns-and-scalds/Pages/Treatment.aspx is the place to go for the best available up-to-date advice on burns.
What matters is that Gattefossé had set the scientific study of aromatherapies going in the right direction. Less than 10 years later he was collaborating with a large number of doctors treating war wounds with Lavender and other essential oils, making sure that methods and results were properly documented, recorded, and co-ordinated; and the science (as well as the practice) of aromatherapy continues to develop in the ways he so strongly influenced.
 [Oleochemistry: the study of vegetable oils and animal oils and fats, and oleochemicals derived from these fats and oils or from petrochemical
feedstocks through physico-chemical modifications or transformation. First used in the making of soaps, oleochemistry is now part of our daily lives where it is found in a wide variety of sectors like food, cosmetics, pharmaceutical and industrial.]  http://www.gattefosse.com/
 Gattefossés Aromatherapy – The First Book on Aromatherapy
Author: René-Maurice Gattefossé
Publisher: C.W. Daniel Company
Copyright: 1993 (Translated from Original 1937 French Text)