94 REPORTS ON THE STATE OF SCIENCE.—1917. 
temperature, the bivalent metals acting more strongly than univalent 
metals in equal concentrations. 
The nitrates of the metals have a greater influence than the chlorides. 
The addition of electrolytes also causes the gelatin to dissolve in mixtures 
of non-electrolytes and water, e.g., alcohol or acetone. On removing the 
salt by dialysis or by precipitation the gelatin separates. 
The effect on gelatin of a large number of reagents, including acids, 
salts, phenols, &c., is described by A. L. Lumiére and A. Seyewetz (* Bull. 
Soc. Chim.’ 1908, 3, 743-750). 
Certain salts, e.g., those of Va, Ni, Co, and Cr, render gelatin insoluble 
(Liippo-Cramer, * Zeitschr. Chem. Ind. Kolloide,’ 1909, 4, 21-23). Salts of 
Al and especially FeCl, have a similar effect (C. E. Millar, ‘ Jour. Soc. 
Chem. Ind.’ 1900, 326). 
With regard to the effect of chromium salts on gelatin, the action 
depends upon the constitution of the chromium compound employed. 
Thus when a small quantity of bichromate of potash is added to gelatin it 
has no sensible effect except after exposure to light, when the organic 
colloid becomes insoluble in hot water. The action of the bichromate is 
not well understood, although it is regarded as an oxidation process. The 
reaction, however, 1s made use of to a very large extent in photography, 
especially in the photomechanical processes. Dealing with gelatin from 
this point of view, C. W. Gamble (‘ Jour. Soc. Chem. Ind.’ 1910, 65) states 
that it is a complex, the parent substance being ‘ collagen.’ After heating 
for some time, gelatin passes into ‘gelatose ’ which does not gel, this change 
being also brought about by bacterial action and by proteolytic enzymes, 
especially trypsin. Bone gelatin and fish glue are classed as gelatoses. 
Gelatin and gelatose are precipitated by bromine in acid solution (A. H. 
Allen, ‘ Analyst,’ 1897, 258) and also by chromic acid. Gelatin peptone is 
not precipitated by chromic acid, nor is it rendered insoluble after addition 
of bichromate and exposure to light; it also passes freely through a 
membrane. Gelatin peptone has a marked effect upon gelatin causing a 
greater tendency to dispersion. Glucose has a similar effect. Both these 
substances counteract the effect of bichromates after action of light, the 
film tending to dissolve on washing. 
Gelatin has now almost entirely replaced albumin and collodion in the 
manufacture of photographic plates and papers. It is more reliable and 
more easily penetrated by the solutions employed ; the introduction of this 
colloid for these purposes entirely revolutionised the photographic trade, 
and is largely responsible for the enormous developments which have taken 
place. 
The preparation of photographic emulsions is based on the knowledge 
gained after years of practical work, and may be regarded as an industry 
founded upon the application of capillary phenomena. 
There are many interesting points in connection with photography ; for 
instance, Liippo-Cramer (* Zeitschr. Chem. Ind. Kolloide,’ 1907, 2, 171-172) 
found that silver oxide formed in a gelatin film by the action of NaHO 
caused the gelatin to become opalescent and insoluble in boiling water. 
It is proposed to purify gelatin for photographic purposes by means 
of electro-osmosis, the fat and electrolytes passing through the membrane 
while the albuminoid substances are precipitated (Ges. fiir Elektro- 
Osmose, Eng. Pat. 21,448, and 21,484, 1914). 
Electro-osmosis is likely to play an important part in industry in the 
