RESPIRATION. 60 ( J 



animal which is made to breathe them. Gaseous bodies, as far 

 as respiration is concerned, may be divided into two classes : I. 

 Unrespirable gases ; II. Respirable gases. 



I. The gases belonging to the first class are of such a nature 

 that they cannot be drawn into the lungs of animals at all ; the 

 epiglottis closing spasmodically whenever they are applied to it. 

 To this class belong carbonic acid, and probably all the other 

 acid gases, as has been ascertained by the experiments of Pilatre 

 de Rozier.* Ammoniacal gas belongs to the same class ; for 

 the lungs of animals suffocated by it were found by Pilatre not 

 to give a green colour to vegetable blues. f 



II. The gases belonging to the second class may be drawn 

 into the lungs, and thrown out again without any opposition from 

 the respiratory organs ; of course the animal is capable of re- 

 spiring them. They may be divided into four subordinate clas- 

 ses; 1. The first set of gases occasion death immediately, but 

 produce no visible change in the blood. They occasion the ani- 

 mal's death merely by depriving him of air, in the same way as 

 he would be suffocated by being kept under water. The only 

 gases which belong to this class are hydrogen and azotic. 2. 

 The second set of gases occasion death immediately, but at the 

 same time they produce certain changes in the blood, and there- 

 fore kill not merely by depriving the animal of air, but by cer- 

 tain specific properties. The gases belonging to this class are 

 carburetted hydrogen, sulphuretted hydrogen^ carbonic oxide, and 

 perhaps also nitrous gas. 3. The third set of gases may be 



* Jour, de Pbys. xxviii. 418. Pilatre de Rozier went into a brewer's tub 

 while full of carbonic acid gas evolved by fermentation. A gentle heat mani- 

 fested itself in all parts of his body, and occasioned a sensible perspiration. A 

 slight itching sensation constrained him frequently to shut his eyes. When he 

 attempted to breathe, a violent feeling of suffocation prevented him. He sought 

 for the steps to get out ; but not finding them readily, the necessity of breath- 

 ing increased, he became giddy, and felt a tingling sensation in his ears. As 

 soon as his mouth reached the air he breathed freely, but for some time he could 

 not distinguish objects ; his face was purple, his limbs weak, and he understood 

 with difficulty what was said to him. But these symptoms soon left him. He 

 repeated the experiment often ; and always found, that, as long as he continued 

 without breathing, he could speak and move about without inconvenience ; but 

 whenever he attempted to breathe, the feeling of suffocation came on. Ibid, 

 p. 422. 



f Jour, de Phys. xxviii. p. 424. 



f See Chausier's experiments, ibid. Ivi. p. 35. 



Qq 



