Sir Humphry Davy learned quite a bit from experiment in which he exposed animals — including himself — to laughing gas. One of the things he learned was how to look at blood and determine a cause of death. (Usually that cause of death was exposure to Sir Humphry Davy.)
Sir Humphry Davy was a British chemist best known for experimenting on himself, sometimes with suicidal recklessness. Through self-experimentation he learned a lot about nitrous oxide, and helped pave the way for anesthesia. He did not, however, confine himself to self-experimentation. In researching laughing gas, he performed a series of experiments that, while legitimate experiments, do look grisly to modern eyes. Even Davy saw the grimness of them. He referred to his assistant as “a murderer of cats and dogs.”
So what can we learn by murdering various things? Davy learned that organisms sealed in a case with pure nitrous oxide died twice as quickly as they did if they were exposed to pure hydrogen or were submerged in water. This included fish, which indicated to Davy that fish might have a need for oxygen comparable to the need humans had for oxygen. This sounds obvious now, but at the time, it seemed to scientists (including Davy) that overexposure to oxygen, as well as temperature change, might be what killed fish when they were taken out of the water. Davy did experiments during which he put fish in deoxygenated water (which had been boiled and cooled), water saturated with hydrogen, and water saturated with nitrous oxide. They behaved just the way land animals did, and died — but they died more slowly when their water was saturated with laughing gas.
He could also see that the stuff we breathe goes from our lungs to our blood. Autopsies on the animals showed him that the blood of animals exposed to different gases was different colors. He decided to simplify (and de-fatalize) the experiment with human volunteers. He had people bleed into vials filled with different gases, and observed the color that the blood turned in each vial.
His experiments culminated in a kind of bizarre collection of color swatches. To be able to show people the color of blood exposed to different gases (or recovered from bodies exposed to different gases), he took each of the drops of blood and squeezed it between plates of glass. It was a move reminiscent of both modern laboratory slides and pressed-flower decorations. By searching through the Davy slides, one could see if a person had died under the influence of plain old oxygen (in which case their blood was ochre) or nitrous oxide (in which case their blood was crimson).