On Sal Nitrum and Nitro- Aerial Spirit 73 



same level as the water outside, as may be done by 

 means of the bent syphon already described. When 

 this is done let the water outside be drawn off a little 

 in order that the height of the water within may be 

 better observed. And let it be indicated by papers 

 attached here and there to the sides of the glass. And 

 so you will soon see the water sensibly rising into the 

 cavity of the glass, although the heat produced by 

 the presence of the animal in the glass, and also 

 the breath proceeding from it, might be expected rather 

 to produce an opposite effect. 



But we can perceive in the following way the extent 

 to which the air enclosed in the glass undergoes con- 

 traction before it becomes unsuitable for sustaining 

 animal life. For let the space in the glass occupied by 

 the air when the animal was at first placed in it and 

 also the space occupied by the same air when the 

 water has risen in the glass after the suffocation of the 

 animal be measured, as can be done by pouring water 

 into those spaces so as to fill them and measuring it — 

 but warning should be given here in passing that when 

 these spaces are thus measured everything should 

 remain in the glass the same as before. And now let 

 it be ascertained by calculation how much the first 

 space is greater than the second. For to that extent 

 the air is lessened as to its elastic force and volume by 

 the breathing of the animal. And in fact I have 

 ascertained from experiments with various animals 

 that the air is reduced in volume by about one- 

 fourteenth by the breathing of the animals. But 

 care should be taken in making this experiment that 

 the animal be placed only a little above, the surface of 

 the water, for a reason to be afterwards given. 



From what has been said it is quite certain that 

 animals in breathing draw from the air certain vital 



