PRACTICAL EXERCISES 295 



chamber, sealing 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. 



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

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



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

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

 grin of the soda-lime tubes in carbon dioxide. 



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

 correspond, the gain being always greater th?.n the loss. The surplus 

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

 to the hydrogen and carbon of its substance to form water and carbon 

 dioxide. Calculate the respiratory quotient (p. 242 ). 



11. Muscular Contraction in the absence of Free Oxygen (see 

 p. 261). (i) Pith a frog (brain and cord). Cut off one hind-leg 

 at the middle of the thigh, 'and strip the skin from it. Pass a 

 thread under the tendo AchUlis, tic it, and divide the tendon below 

 it. Free the tendon and the* gastrocnemius muscle from the loose 

 connective tissue lying between them and the bones of the leg, and 

 divide the latter just btlow-the knee. Remove superfluous thigh 

 muscles, and fasten the gastrocnemius in a moist chamber by means 

 of the femur. Attach the thread on the tendon to a lever. Connect 

 the poles of the secondary coil of an induction machine by fine copper 

 wires to the femur and the tendon. Put a battery and simple key in 

 the primr.ry, and arrange it for single shocks. Stimulate the muscle 

 and observe the height of the contraction. Now pass into the chamber 

 a current of washed hydrogen gas from a bottle containing granulated 

 zinc, upon which a little dilute sulphuric acid is poured from time to 

 time. The air in the moist chamber will soon be entirely displaced 

 by the hydrogen. Nevertheless, the muscle will contract on being 

 stimulated as before, and the stimulation can be repeated many times. 



12. Oxidising Ferments. Wash out the bloodvessels of a dog or 

 rabbit (Practical Exercises, p. 57). Chop up finely portions of 

 pancreas, spleen, muscle, lungs, and kidney, keeping each separate, 

 and avoiding any contamination of one by another. Grind up half 

 of each portion with sand in a small mortar, and extract with a small 

 quantity of water, keeping all the extracts separate. Into each of 

 eleven test-tubes put 10 c.c. of a colourless dilute alkaline solution of 

 paraphenylenediamin and a-naphthol (freshly made by mixing solu- 

 tions of the two substances in equimolecular proportions* and adding 

 a little sodium carbonate). To five of the tubes add the chopped 

 organs, to five the watery extracts of the organs, and enough water 

 to make the volume equal in all the tubes. To the remaining tube 

 add the same amount of water. Observe in which tube a change of 

 colour takes place (p. 265). 



* I.e., the weight of each of the two substances in the mixture should be 

 proportional to its molecular weight. 



