—— 
4 
30 REPORTS ON THE STATE OF SCIENCE.—1917. « . . 3 
from the fibres by the digestive effects of bacterial enzymes, and complete 
the emulsification of fats and the solution of residues of the epidermis. 
It is obvious that the attainment of all these varied results by an arti- 
ficial preparation is no easy matter, but an approach to a complete solution 
has been made by J. T. Wood" (followed by Dr. Réhm, who has 
improved working details), by a mixture of ammonium chloride and 
pancreatic digestive ferments, which for many purposes fulfils its object 
better and much more safely than the old materials. The tryptic ferments 
dissolve the epidermis residues and cement-substance, but scarcely 
affect the collagen fibres (v.s.). They also facilitate emulsification 
of fats by reducing the surface tension between jelly matters and the 
liquid, while the presence of free ammonia and excess ammonium salts 
regulates the hydroxyl concentration to something near the alkalinity 
required for minimum swelling (v.s.). Possible improvement lies 
in the direction of the discovery of new enzymes, and of suitable weak 
bases and ‘ buffer’ substances, to give the precise degree of solution and 
of acidity or alkalinity required for the various leathers. For firmer 
leathers the use of weak acids regulated by excess of their salts pro- 
duces a sufficient degree of neutralisation and flaccidity. 
We must now consider the conversion of the still raw and very putre- 
scible skin into permanent leather. We have seen that this can be accom- 
plished by dehydrating the fibrils without allowing them to adhere (v-s.), 
and by coating them with water-resisting substances; but it is known 
that similar effects of an even more permanent character can be produced 
by reagents (notably formaldehyde and bromine) which act chemically 
on the collagen fibre, rendering it insoluble in water, but which in their 
nature cannot deposit any exterior coating such as was assumed by 
Knapp. We must therefore conceive the process as being in most cases 
a combination of both chemical and physical effects, of which sometimes 
one, sometimes the other, preponderates, according to the method employed. 
We have also to consider reactions which from their colloid character 
differ somewhat widely from those of free ions to which the term * chemical ’ 
is generally applied. It is therefore best to proceed from simple cases 
of which definite explanation can be given, to the complex in which 
more than one sort of reaction takes place. 
Knapp’s alcohol leather, in which a material is produced with all the 
physical characteristics of a complete leather by simple dehydration of 
the hide fibres under conditions preventing adherence, has been already 
mentioned (v.s.). The theory of acid swelling has also been described, and 
it has been shown that as the anion concentration of the external solution 
is increased the difference of osmotic pressure between it and the jelly 
which causes swelling is diminished without limit, and the fibre contracts 
by its internal attractions. This fact is applied in the process of ‘ pick- 
ling’ which is principally employed in the preservation of sheepskins 
betore tanning. The skins, after unwooling, are treated in a bath of 
dilute acid, generally sulphuric, to which some salt is added to prevent 
excessive swelling, and are then transferred to a saturated solution of 
common salt. The dehydration of the fibre is very great—the skin 
becomes thin and flat, and can be preserved almost unlimitedly in the 
wet condition; and if dried out and loosened by a little mechanical 
1° Wood, Puering, Baling and Drenching, Spon, 1912, p. 186. 
