SUCTIONAL RESPONSE AND ASCENT OF SAP 369 



seen how the normal rate of suction and its variations may 

 be recorded accurately. The next difficulty to be overcome is 

 that of introducing the changed conditions without creating 

 an}' disturbance, thus practically maintaining the continuity 

 of record. It will be necessary to observe, among other 

 things, the immediate and after-effects of cold and heat, as 

 well as those of various chemical reagents. This is accom- 

 plished by the right manipulation of the four stop-cocks. 

 Let it be supposed that we wish to study the immediate 

 and after-effects of cold. Up to this time the stop-cocks 

 A and a' have been opened — B and B' being closed — and the 

 balanced horizontal record taken. For the sake of simplicity 

 I refer to the stop-cock A' as the only one which opens and 

 closes the communication with the compensator c. In reality, 

 however, there is a second stop-cock in its neighbourhood, 

 by which the balancing adjustment is first made, A' being 

 employed for opening or closing communication under such 

 an adjustment. The reservoir R is now filled with cold 

 water ; A and a' are next closed — thus arresting the water- 

 index— and B and b' opened. The water then leaves the 

 plant-vessel by the overflow pipe, and its place is taken by 

 cold water from R. After this, the stop-cocks B and P.' are 

 once more closed, and A and a' opened. The index, being 

 now released, indicates by its movement the excitatory 

 or depressing effect of cold. It must be remembered that 

 the index was previously adjusted to balance. Should the 

 effect of cold prove to be excitatory, the rate of suction 

 would be increased ; under-balanced by the supply of water 

 from c, the index would move to the right, thus giving 

 positive response. If, on the contrary, the effect should be 

 depressing, the rate of suction would be decreased, and the 

 water from the compensator would produce an over-balance, 

 causing a movement of the index to the left, or negative 

 response. By now filling the reservoir R with water at 

 the ordinary temperature, and repeating the operation, the 

 original condition is re-established, and the effect of re-esta- 

 blishment of old conditions observed. In a similar manner, 



B B 



