PRACTICAL EXERCISES 277 



quantity of water given off by the animal in an hour ; the difference 

 in the combined weight of 4 and 5, the quantity of carbon dioxide. 

 Weigh the cat, and calculate the amount of water and of carbon 

 dioxide given off per kilo per hour. 



(2) For the student it is more convenient to use smaller animals. 

 The mouse may be taken as an example of a warm-blooded animal, 

 and the frog of a cold-blooded. Instead of the Woulff's bottles use 

 wide test-tubes connected as in Fig. 102, and for the animal chamber 

 a small beaker, closed with a very carefully-fitted cork which has 

 been boiled in paraffin. The inlet and outlet tubes of the chamber 

 are to be introduced through this cork. The holes for these are to 

 be bored with the greatest care, and the tubes to be put in while the 

 cork is still hot from boiling in paraffin. Also insert a thermometer 

 about 6 inches long registering from o C. to 45 C. Modeller's wax 

 is to be used finally to render all the junctions air-tight. 



Add to the series of tubes described in the apparatus a single 

 tube containing baryta-water. This is placed after the tube 5, and 

 so arranged that the air-current bubbles through the water. As long 

 as the absorption of carbon dioxide is complete, the baryta-water 

 remains clear. Beyond this a water-bottle should be placed to act 

 as a valve and to indicate the negative pressure in the apparatus. 

 It can be most simply constructed by using a cylinder of stout glass 

 tubing in a wide-mouthed bottle containing some water, the inlet and 

 outlet tubes passing through a paraffined cork which seals the upper 

 end of the cylinder. 



Before making an observation, test whether the apparatus is air- 

 tight, as explained above, after introducing the animal into the 

 chamber, seating the latter with wax, and connecting it with the 

 absorption tubes. But a negative pressure of 2 or 3 inches of water 

 is a sufficient test for the small apparatus. 



To make an observation, set the air-current going at the desired 

 rate. Allow it to run for a few minutes till the carbon dioxide, which 

 has accumulated during the testing, has been swept out. At a time 

 which has been decided on and noted, stop the current by discon- 

 necting the water-pump. Disconnect and stopper up the animal 

 chamber, and weigh it as quickly as possible. Connect up again, 

 using only recently-weighed absorption-tubes, and finally connect 

 with the water-pump and allow the current to pass for a definite 

 period, say an hour. If a consecutive series of observations is to be 

 made, two sets of tubes should be prepared for use during alternate 

 periods. Use in each case two soda-lime tubes, the most recently 

 filled one being placed second of the two. 



The soda-lime should not be too dry, or absorption is not 

 sufficiently rapid. The following facts are made out in the observation : 



(a) The loss of weight by the animal chamber (chiefly loss by the 

 animal), (b] The gain of the sulphuric acid tube in water, (c) The 

 gain of the soda-lime tubes in carbon dioxide. 



If we compare total loss and total gain, we find they do not corre- 

 spond, the gain being always greater than the loss. The surplus can 

 only be oxygen which has been absorbed by the animal and added to 



