THE MIcROSCOPE. 323 
and contained the heavy sediment of the rapidly increasing 
yeast. It would be desirable, if one could control the conditions, 
to keep tubes in several ovens held constantly at graduated tem- 
peratures from 40° to 80° to see what heat is most favorable, but 
this the class could not attempt. 
The third series of experiments was undertaken to learn 
whether light and darkness affected the growth. These were 
only roughly true, for no attempt was made to keep the tem- 
perature condition of the two tests constant. For comparison 
some fresh water Alga growing abundantly in Sach’s solution 
was kept in a tube with each of two tubes of yeast in Pasteur’s 
fluid, and one pair kept in the diffused light of the room, the 
other in ja dark place. In every examination the yeast in the 
dark was found to be thriving, while the color was gradually 
fading out of the Alge placed in the dark. 
The fourth series of experiments was most elaborate. The 
problem was to test for the amount of various poisons, the 
presence of which would prove fatal to yeast. Final results 
were not reached in every case because of the lack of time, but 
interesting and trustworthy results were in all cases obtained. 
The substances tested were carbolic acid, corrosive sublimate, 
acetic_acid, hydrochloric acid, sulphuric acid and alcohol. These 
sorts of tests can of course be indefinitely extended. In each 
case one part of a given strength of the poison was added to eight 
parts of Pasteur’s fluid and one part of sugar, and the resultant 
fluid was of course one tenth weaker than the trial fluid. The 
carbolic acid set was made by taking warm, hence fluid, carbolic 
acid, and weakening it to the desired dilution. The saturated 
aqueous solution of corrosive sublimate was employed. The 
alcohol.was absolute; the hydrochloric and sulphuric acids were 
the ordinary commercial articles, and the acetic acid was glacial. 
The results with each reagent were carefully tested and corrobo- 
rated; both external, naked eye appearance of the fluid in the 
tubes, and microscopic examination of the colonies as luxuriant 
or scanty in budding, as well as the appearance of the individual 
yeasts, were taken in judging whether a particular fluid was one 
favorable to growth. The results of these experiments were as 
follows: 
1. Carbolic acid. The presence of this substance in amounts 
as high as 1, per cent. is perfectly harmless. The yeast cells 
are large and translucent, and in thriving colonies; and in ad- 
