ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 97. 
small quantity of borax entirely inhibits the precipitation of gelatin by 
tannin, but boric acid does not influence it. Gelatin which has been 
precipitated by alcohol contains the whole of the mineral matter, and is, 
in fact, practically unaltered by the treatment. 
In addition to the patents in connection with gelatin mentioned in the 
text the following are of interest :— 
J. A. Swan (Eng. Pat. 3,305, 1866) claimed the use of chrome alum 
and other salts of chromium for rendering gelatin insoluble. 
E. M. Knight and A. H. Hobson (Eng. Pat. 13,168, 1887) prepare a 
cement or gum by heating glue or gelatin with soda. 
A. Zimmermann (Chem. Fabrik auf Actien vorm. E. Schering, Eng. Pat, 
2,036, 1894) patented the application of formaldehyde for rendering gelatin 
_ and isinglass insoluble. The same firm claimed that the addition of a very 
small quantity of formaldehyde to glue materially increased its strength 
_ without rendering it insoluble (Eng. Pat. 4,696, 1894). J. Hofert (Eng. 
Pat. 4,697, 1894) uses gelatin as a waterproofing agent, rendering it 
insoluble with formaldehyde. An artificial silk (Vandaura silk) is manu- 
 factured from gelatin threads rendered insoluble by formaldehyde (A. 
_ Millar, Eng. Pat. 6,700, 1898; see also A. Millar, ‘ Jour. Soc. Chem. Ind.’ 
1899, 17, and C. E. Millar, zbid., 1900, 326). 
E. J. Mills (Eng. Pat. 8,847, 1895) proposed to use alkali and microbes, 
such as Bacillus liquefaciens, and liquid glue. Plastic substances are 
obtained by digesting ‘ ossein’ in NaHO solution (A. Heilbronner and 
H. A. Vallee, Eng. Pat. 20,548, 1906). 
A mixture of gelatin, glycerine, formaldehyde (trioxymethylene), 
and an oxidising material, is employed for moulding purposes (B. Sauton, 
Eng. Pat. 27,616, 1906). 
W. H. Perkin and Whipp Bros. & Tod (Eng. Pat. 23,030, 1907) claim 
the use of coal-tar creosote for dissolving gelatin. 
The coagulation of colloids, e.g., gelatin, glue, agar, casein, starch 
(commercial dextrin), is prevented by treatment with a salt of an organic 
sulphonic acid or derivative (The Arabol Man. Co., Fr. Pat. 394,173, 1908). 
Use of levulose for softening gelatin and glue (F. Evers, Eng. Pat. 
25,145, 1909). See also A. H. Church, ‘ Chemistry of Paints and Painting,’ 
p. 72. 
COLLOIDS IN THE SETTING AND HARDENING OF CEMENTS. 
By Dr. C. H. Descu, Metallurgical Laboratory, University of Glasgow. 
The most important chemical investigation of the setting and hardening 
of calcareous cements is that which was submitted by Henry Le Chatelier 
‘in 1887 as a thesis for the Doctorate in the University of Paris. The 
setting of plaster of Paris was there shown to be due to the formation of 
an unstable solution of the hemihydrate in water, soon followed by the 
crystallisation of the less soluble dihydrate in spherulitic forms, the 
strength of the plaster after setting being due to the interlacing of crystals 
from neighbouring centres. Later investigations have only confirmed 
the truth of this explanation. In the case of Portland cement, it was 
hown that the basic silicates and aluminates of calcium contained. in the 
cement clinker were hydrolysed by water, forming less basic salts and 
free calcium hydroxide. The most stable of these salts were determined 
1917. H 
