626 
POPULAE SCIENCE EEVIEW. 
tliree hours. When the exposure has been deemed sufficient, 
the picture is then acted on with rectified tm*pentine containing 
a small quantity of alcohol. This substance dissolves away all 
the asphalt which has not been exposed to the sun, and in this 
way leaves a perfect positive print in asphalt upon the stone. 
When the stone has become dry, the usual lithographic opera- 
tions, such as etching, &c., may be proceeded with. There is 
one serious objection to the above process, and that is, that 
the defects — whatever they may be — of the negative are 
transferred at once to the stone, and hence, if the picture be 
an unsatisfactory one, the stone must be washed and re-ground. 
The mode known as the transfer process is, therefore, the one 
more generally employed in these countries. This simply 
consists in transferring the image first taken to paper prepared 
for the purpose, the latter being then pressed upon the stone. 
The transfer paper is of various kinds, but one of the simplest 
is that in which the paper, which is to be exposed beneath the 
photograph already taken, is prepared with gelatine and 
bichromate of potash, and coated with fatty ink, the latter 
being aftersvards easily washed off with water from those parts 
unaffected by light. In all processes of photo -lithography the 
great difficulty is the production of the negatives. These 
must be of the greatest excellence, for the slightest defect in 
the half-tones wiU be terribly exaggerated in the print. In- 
deed this is so true of photo-lithography that there is a certain 
class of negatives, such as those of animal structures, contain- 
ing an immense combination of half-tints, which cannot be 
applied to stone with anything like successful results. 
There is a twin-brother of photo-lithography which is not 
yet as great a favourite nor nearly so useful a member of 
industrial society ; we refer to the process of applying photo- 
graphy to engraving. The method of producing engravings 
by lithography has not yet been very successful, though it has 
produced some wonderful and not to be anticipated results. One 
of the processes has received the name of ]p^^oto-galvanograp}iy , 
a very sesquipedalian epithet it must be owned, but still one 
which expresses the meaning of the process tolerably well. It 
is essentially a combination of a peculiar form of photography 
with electrolysis, or the deposition of metals by means of 
galvanic electricity. Of course, in this instance, as in the 
former ones, a negative of as perfect a kind as possible must 
be taken, and this is placed over a plate of glass which con- 
tains a layer of sensitized material of a particular kind. This 
sensitized substance is not collodion, but is a compound of 
gelatine, water, and concentrated solution of bichromate of 
potash. The effect of exposing a layer of this preparation 
beneath a negative to the action of the sun is not only to 
