or Art of multiplying Daguerreotypes. 367 
There is a substance in the market, which* goes under the 
name of Cooper’s Isinglass, which I have found much better 
than any other for these purposes. 
16. The plate is to be arranged horizontally, with its face 
upwards, on some proper support, in the current of hot air 
that rises from a stove. The isinglass is to be poured on until a 
stratum about {th ofan inch deep is upon the plate. It is then 
suffered to dry, the process being conducted so as to occupy 
two or three hours. When perfectly successful, as soon as 
the drying is complete, the film of isinglass now indurated into 
a tithonotype splits off, and on being examined either by re- 
flected or transmitted light will be found to bear a minute 
copy of the original. 
17. To return for a while to the theory of these images. 
Whilst thus it is plain that the optical effect depends on sur- 
face configuration alone, and does not seem to have any im- 
mediate relation to the thickness or thinness of a film, it is 
very different with the chemical effect on which the whole 
phzenomenon depends. 
18. The Daguerreotype film, which has been under the in- 
Jluence of light, is polarized throughout its structure previous to 
mercurialization. 
19. I use the word “ polarized” in its chemical sense. An 
illustration will serve to show the signification I attach to the 
term. When water is placed between platina electrodes its 
oxygen is liberated from one of them, and its hydrogen from 
the other, and the intervening liquid assumes a polar state, a 
series of decompositions and combinations going on. As that 
water is polarized, and undergoes polar decomposition, so too 
do the same phzenomena hold in the case of the Daguerreo- 
type film. 
Ist. We know that no iodine is ever evolved from the plate, 
even under the most prolonged action of the light. (Phil. 
Mag. Sept. 1841, p. 201.) 
2nd. ‘The cause of the final appearance of the image is due 
to silver being liberated on the anterior face of the plate. (Sept. 
1841, p. 201.) 
3rd. When, by the action of gelatine, the iodine and mer- 
cury are both removed from the plate, it is obvious that the 
plate has been corroded wherever the light fell. Iodine there- 
fore has been evolved on the posterior face of the film, and 
is the cause of this corrosion. 
20. From the circumstance, therefore, that iodine is evolved 
at the back of the film and silver at its front, and the film itself 
remaining the same in thickness throughout, it is obvious that 
there is a strong resemblance between this phenomenon and 
