1899] TOXIC EFFECT OF DELETERIOUS AGENTS 299 
needs no further comment. The results with. Penicillium and 
other forms abundantly confirmed the test, and lead to the con- 
clusion that when water is used at the bottom of cells containing 
vapor pressures are being tested, erratic germinations are to be expected, 
and in the case of a test of any substance —particularly those of high 
w low vapor pressure —the toxic properties shown will be less than a 
correct method would indicate. 
In the regular work reported in this paper, a few drops of the 
same solution as that used in the hanging-drop of any particular 
cell was in every case placed in the bottom of that cell, thus 
establishing complete equilibrium of vapor pressures in the cells, 
and thereby preventing changes in the concentration of the solu- 
tion: under test as must take place when this precaution is not 
observed, By this method it was found quite feasible to make 
hanging-drop cultures of any composition: whatever, ranging 
from ‘— (70 per cent.) solution of alcohol to a a (21 per 
‘nt. solution of potassium hydroxid. 
Stock. solutions of chemicals.—These were prepared in all cases 
; by a responsible chemist from the purest chemicals obtainable. 
It was found desirable to have stock solutions in highly con- 
. *entrated form so that all necessary diluting could be made by 
_-Wding beet infusion, The stock solution of HCl, for example, 
Sntained 25 grams pure HCl per too®. HCI contains 3.58 
oe cent. HC] by weight. Such a solution was gotten by taking 
: ¥ of the stock solution and diluting, by adding beet infusion, to 
a. 2 Was gotten by diluting 10° to 20°, and so on. 
Chemical a 
nal and y egular cultures—Inasmuch as the work was in most 
hanging-drop cultures and solutions of substances having very high | 
