288 
POPULAR SCIENCE REVIEW. 
Mr. Hall, of Bury, stands foremost in this division ; and his 
Brussels’ carpet loom is remarkable for the simple, in- 
genious, and effective application of the magnet for drawing 
over into position for re-inserting the weft wires which keep the 
loop or 44 pile ” up during the process of weaving. 
Space warns me to turn to the 44 Inventions.” Some, indeed 
most of them, are well known to those who stand to the front of 
general knowledge ; but to the great majority of mankind 
they will be at least novelties previously only heard or read 
of. The most notable of these, for any general and wide-spread 
practicable purpose in ordinary daily life, is that special appli- 
cation of photography for book illustrations called 44 heliotype ” 
which we have ourselves used for the production of our illus- 
trative plate. The process is a very simple but most effective 
and practical one. 
Of all the shortcomings of all previous processes and attempts 
in this direction — the application of photography to book-work 
— the most serious and vexatious, and the most obstructive to 
commercial purposes, has been the dependence upon sunshine. 
Silver prints, each one printed by a separate act involving a 
separate and uncertain interval of time, with results in the 
pictures produced the very opposite of uniform, were found to 
be utterly impracticable. Then followed a series of very inge- 
nious and, in some cases, very beautiful methods, but in all 
which there have existed obstacles to production in commercial 
quantities and with requisite commercial rapidity and uni- 
formity. This has been perfectly accomplished by heliotype — 
the prints being by it printed direct from the ordinary printing- 
press by ordinary printing-ink. The impressions are thus as 
rapidly taken off as lithographs, and are as permanent and as 
uniform as the most approved classes of book or plate-printing. 
The process may be easily understood. Chromate of potash is 
sensitive to light, hardening in proportion to its intensity and 
the time of exposure ; chromalum renders gelatine insoluble. 
Gfelatine, when mixed with chromalum and chromate of potash, 
becomes a compound sensitive to light and insoluble in water. 
It is not, however, impervious in the mass ; but the parts har- 
dened by light are so. If, then, a photographic negative taken 
by the camera be placed over a thin sheet of this prepared gela- 
tine, an invisible picture is hardened into its very substance, 
and this picture being impervious to water in proportion to 
the degree of hardening, rejects water like the greasy drawing 
on a lithographic stone ; the mass of the gelatine plate, however, 
absorbs the water like the body of the lithographic limestone ; 
and so, just as in lithographic printing, the picture takes the 
printers’ ink from the printing-roller, whilst the plain water- 
