SCIENCE AND INDUSTRY. 
the formula Cr (OH) SO y. Since chrome liquors are usually made 
by reducing sodium dichromate, they also contain a considerable amount 
of sodium sulphate. Sometimes the hides are taken from the pickle 
bath and put into a separate tanning bath and sometimes the chrome 
preparation is added directly to the pickle liquor containing the hides, 
After the hides have been drummed or churned in the chrome liquor 
for a day or more, the green colour of the chrome will have penetrated 
them completely, and they are then tested to determine whether or not 
the tannage is complete. This is done by keeping strips of the leather 
in boiling water for five minutes or longer; if they are fully tanned, 
the boiling water will apparently be without effect upon them, but any 
_ unchanged collagen present will be converted into glue, causing a con- 
siderable distortion of the strips. When the hides are not fully tanned 
at this stage, it is generally necessary to reduce the acidity by a cautious 
addition of alkali. 
ae > 
CompLexiry oF Curome Tannine Process. 
Tt would seem that tanning with inorganic salts and acids should be 
less complex than vegetable tanning, but the process is nevertheless 
almost bewildering in its complexity. Work now in progress at Colum- 
bia University has already shown that chrome liquors are much more | 
~~ Surface ~ 
COWHIDE FROM SOAK VATS (x 1%). 
complicated systems than we previously had reason to believe, although 
any one who has done much experimenting with chrome tanning must 
have experienced the annoyance of not being able to duplicate certain 
results because of the variation of some unknown and therefore wncon- 
trollable factors. : 
A. W. Thomas and his collaborators at Columbia have shown by 
hydrogen electrode measurements that the acidity of a chrome liquor 
changes with the time, especially just after some change has been made 
in the liquor, such as dilution or the addition of acids or alkalis. The 
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