=) TOXIC EFFECT OF DELETERIOUS AGENTS 295 
} pressure may be very great, as is the case with KOH and 
H,SO,. On the other hand, should the substance be quite vola- 
tile the increased vapor pressure of the substance added may 
_ more than counterbalance the lowering of the vapor pressure of 
the water, as is the case when ammonia or alcohol is mixed with 
water. To recapitulate in brief, the addition of any substance 
_ orsubstances to water gives a mixture with a vapor pressure at 
_ ‘ariance with that of pure water. This vapor pressure may be 
_ eater or less than that of pure water, depending on the physi- 
_ tal properties of the substance or substances added. Thus, a 
: hanging-drop containing KOH in a cell in which water has been 
; placed below (as is the usual method), absorbs moisture by rea- 
_ Son of its low vapor pressure. Indeed, there will be a constant 
_ distillation of water vapor from the water below to the culture 
. drop, until all the water has passed up or the drop, becoming too 
‘ large to “hang,” falls to the bottom of the cell. With alcohol 
the reverse takes place. No sooner is such a culture made up 
than the alcohol begins to distill from the hanging-drop, and it 
fas not, even for an hour, the concentration supposed to be 
Present. Such a culture as a test of toxicity is valueless. 
Were it possible to have all hanging-drops of exactly the 
~ ‘i, and exactly the same quantity of water below, we 
should expect uniformity in results. Such uniformity, however, 
Would be useless, perhaps worse than useless, as it might, pre- 
attention being’ called to the fundamental error of the 
s n 
found that of four cultures of Macrosporium in a Srohee 
wlan 
pe. Metiatic chlorid, three grew and one failed; of 
Cultures 7? 
409600 
five grew and three failed; of ten cul- 
| 204800 four grew and six failed; of four cultures in 
