1914] HOYT—COLLOIDAL METALS “ 195 
examined by ZsIGMoNDy (17) with the ultra-microscopic apparatus 
devised by him. From his results it was calculated that the 
diameters of the particles in the silver solution were about 50-77 
uu. The gold solution contained particles 20-80 uy in diameter, 
the larger ones being separable by filtration. There was a much 
greater range in the size of the suspended platinum particles, but 
by far the greater portion of these were very small and had an 
average diameter of 44 uu. As the solutions employed in the 
present investigation resembled in appearance those described and 
examined by Zstcmonpy, it is probable that the particles in these 
solutions had a similar degree of magnitude. 
The results upon toxicity obtained in the present investigation 
are shown in tables I-IV, which are self-explanatory. In the 
following consideration of the main points the Roman numerals in 
parentheses refer to the tables and the Arabic ones to the experi- 
ment numbers as there given. 
The solution of silver was extremely toxic (I, 1), killing the alga 
within 17 hours in concentrations above 0.045 ppm. (0.05 per cent 
of the solution originally prepared) and injuring many of the fila- 
ments in strengths as low as 0.00225 ppm. (0.0025 per cent of the 
original solution). In filaments thus killed the cell contents were 
often dark and disorganized and were more or less contracted. 
Various substances were added to the colloidal silver solution to 
determine the effect of their presence upon its toxicity. The addi- 
tion of salts to form a o. 5 per cent concentration of Crone’s nutrient 
Solution? to colloidal solutions of silver in concentrations of 0.045 
ag or less (I, 2), produced marked improvement, as did also the 
addition of about 81 ppm. colloidal platinum (I, 3), or of about 0.1 
* animal charcoal (I, 4). Solutions which had previously been 
mjurious or fatal were thus changed into solutions which were 
‘ntirely or almost entirely non-injurious during the period of the 
®xperiments (1—3 days). In such cases the cell contents retained 
their usual healthy appearance. 
€ effect of the gold solution shows that it was much less toxic 
than that of silver. In order to obtain colloidal gold and hold it in 
i. “ng amen contained salts in the following proportions: 1 gr. KNO;; 9.5 
# ©.5 gr. CaSO,; 0.25 gr. Ca;(PO,)2; 0.25 gr. Fe;(PO,)2. 
