ON EUDIOMETERS. 279 



also, or a little later, by Cavendish. These three observers 

 employed for their experiments what was known at the 

 time as nitrous gas, and which is now known as nitric oxide. 

 When nitric oxide is brought in contact with air, or any 

 gas containing oxygen, red fumes are produced, and these 

 red fumes are easily absorbed by water, so that if the 

 experiment be performed over water, and the two gases 

 are mixed, there will be an absorption and diminution of 

 volume due to the disappearance of the red fumes which 

 have been absorbed by water. Let us try this experiment 

 in a way somewhat similar to that in which it was done 

 the first time it was tried. It was necessary to mix the 

 air and the nitric oxide in equal proportions ; and here I 

 have a cylinder which I will invert over water, and which 

 has upon it two strings to mark the height of the water. 

 I first introduce this vessel full of air into the cylinder, 

 and you see the water stands at the level of the first string. 

 I could show you by introducing a second cylinder full of 

 air that it would stand at the height of the second string, 

 but perhaps you will take my word for it. Now I intro- 

 duce an equal volume of nitric oxide as rapidly as possible ; 

 thus we mix the gases together, and allow them to absorb. 

 You see the production of the red fumes. At the first 

 moment there was a slight expansion, due to the heat, 

 but now you notice there is an absorption, the red fumes 

 are gradually disappearing, and the volume of gas is less 

 than the sum of the two original volumes. This absorption 

 was employed by Fontana, by Priestley, and by Cavendish 

 in order to measure the goodness of air. It was supposed, 

 especially by Fontana, that the greater the diminution of 

 volume the better the air was, because it would contain 

 more of this vital principle. However, it was soon shown 

 that differences in manipulation were quite sufficient to 

 throw out the results, and make them discordant. Fontana 

 describes the way in which the gases are to be mixed exactly 

 in two equal volumes, to be agitated for half a minute no 

 more and no. less, with exactly the same amount of energy ; 

 and any difference in these details of manipulation were 

 quite sufficient to spoil the analyses, and make them dis- 

 agree with one another. It was in consequence of this 

 disagreement that it was imagined that air was a body 

 of variable composition. Cavendish, however, very much 



