398 
POPULAR SCIENCE REVIEW. 
It might at first sight appear as if the atoms of bromide of 
silver when used in the emulsified form would be coarse and 
granular, producing a corresponding granularity in the finished 
negative when compared with a film in w 7 hich the decomposition 
was made. But it is a singular fact that when a negative of 
the former kind is subjected to microscopic examination the 
atoms of metallic silver of which the image is composed are 
extremely fine — so fine, indeed, as, with a power of two-thirds 
of an inch, to show like a stain. When a negative taken in 
the usual way — that is, with a silver bath and iron development 
— is examined under the same power, it appears exceedingly 
coarse, the atoms of reduced silver being very large. 
But it is not necessary that the sensitive preparation be kept 
in a fluid form. We have just said that there appears to be no 
limit to the keeping properties of the desiccated pellicle, pro- 
vided it be kept away from the light. A most useful and 
practical application of this fact is, that a traveller can take 
with him to any distant country a small packet of this pellicle, 
either in a pellicular or a pulverulent form ; and in both of 
these states the preparation is now commercially obtainable. 
The advantage of this to the tourist cannot be over-estimated. 
When the photographic visitor to the Continent takes with him 
a supply of dried plates he runs a risk of having them spoilt by 
the examination, in daylight, of the Custom House officials ; 
when he takes with him only a bottle of emulsion this danger 
is reduced to a minimum, especially if the bottle be actinically 
opaque. But by taking with him, instead of these, a supply of 
the sensitive collodion in the form of a coarse powder, he secures 
the maximum of convenience without any risk whatever. To 
prepare a solution for use a certain proportion of this powder 
is added to a mixture of ether and alcohol, in which it is dis- 
solved ; and thus is made a collodion fit for immediate use. 
To prepare this . sensitive collodion, dissolve a hundred and 
thirty-five grains of pyroxyline in fifteen ounces of a mixture of 
ether and alcohol. There must be ten ounces of ether to five 
of alcohol, but both of these may be the most common methy- 
lated kind. In this is dissolved a hundred and eighty grains of 
anhydrous bromide of cadmium. To this is then added a so- 
lution of three hundred grains of nitrate of silver in five ounces 
of methylated spirits, the solution being aided by heat and the 
addition of a little water. It is poured in the collodion gradu- 
ally, with intermediate shaking, and is then allowed to stand 
for several hours ; after which an excess of bromide is added, 
consisting of seventy grains dissolved in an ounce of methylated 
alcohol. After being well mixed and allowed to stand for two 
or three hours an ounce of glycerine is added, and the whole is 
poured out in a flat dish, washed and dried. It is then cut into 
