﻿1903] 



CHEMICAL STIMULATION 



91 



I 



r 



I 



\ 



latter it is not easy to keep the glass tube by which the 



air 



enters 



the baryta water free of BaCO 



It is not difficult to 



keep either kind of tube clean if it is always washed, after using. 



with dilute HCl. Using two absorption tubes with each culture. 



the experiment is continuous, only a few seconds being used in 



switching the currents of air. From 

 the absorption tubes the air passes 

 to another switch board bottle con- 

 taining some H^SO^. A tube directly 

 from one switch board bottle to the 

 other makes it possible to keep air 



passing 



through 



the cultures with- 



out using the absorption tubes; but 

 this was found inadvisable and was 

 largely given up, because when the 

 absorption tube is returned to use, 

 the height of liquid in it slackens 

 the flow of air, and the irregularity 

 so introduced is not immediately 

 overcome by lengthening the siphon 

 by an equal height of liquid. The 

 glass stop cocks to which the air 

 passes next regulate its rate of move- 

 ment. These cocks must be selected 

 by experience, and once they are 

 working well must not be touched. 

 The H^SO^ in the bottles below 

 them prevents their obstruction by 

 the precipitation of water. 



As aspirators I have used bar- 

 rels, and latterly a metal cask hold- 

 ing 200 liters, the vessel always having a lateral bung so that 

 it might lie on its side. With this aspirator it was possible 

 to run air at the speed I chose, usually about three liters per 

 hour, through both cultures for over thirty hours. It was always 

 filled (the filling requiring but few minutes) the last thing at 

 night, and the effect of the interruption was lost in the long 



Fig. I. 



