_Difficulties in making soluble Cotton._--In making soluble cotton
according to the formula given by Mr. Hadow in the _Photographic Journal_,
and again by MR. SHADBOLT in "N. & Q.," I have been subject to the most
provoking failures, and should feel obliged if MR. SHADBOLT or any other of
your correspondents could explain the causes of my failures, which I will
endeavour to describe.
1st. In using nitrate of potash and sulphuric acid, with a certain quantity
of water as given, I have _invariably_ found that on adding the cotton to
the mixture it became _completely dissolved_, and the mass began to
effervesce violently, throwing off dense volumes of deep red fumes, and the
whole appearing of a similar colour. I at first thought it might be the
fault of the sulphuric acid; but on trying some fresh, procured at another
place, the same effects were produced.
Again, in using the mixed acids (which I tried, not being successful with
the other method) I found, on following Mr. Hadow's plan, that the cotton
was also entirely dissolved.
How is the proper temperature at which the cotton is to be immersed to be
arrived at? Are there any thermometers constructed for the purpose? as, if
one of the ordinary ones, mounted on wood or metal, was used, the acids
would attack it, and, I should imagine, prove injurious to the liquids.
At the same time I would ask the reason why all the negative calotypes I
have taken lately, both on Turner's and Sandford's papers, iodized
according to DR. DIAMOND'S plan, are never intense, especially the skies,
by transmitted light, although by reflected light they look of a beautiful
black and white. I never used formerly to meet with such a failure; but at
that time I used always to wet the plate glass and attach the paper to it,
making it adhere by pressing with blotting-paper, and then exciting with a
buckles brush and dilute gallo-nitrate. But the inconvenience attending
that plan was, that I was compelled to take out as many double slides as I
wished to take pictures, which made me abandon it and take to DR. DIAMOND'S
plan of exciting them and placing them in a portfolio for use. I imagine
the cause of their not being so intense is the not exposing them while wet.
A bag made of yellow calico, single thickness, has been recommended for
changing the papers in the open air. I am satisfied it will not do,
especially if the sun is shining; it may do in some shady places, but I
have never yet seen any yellow calico so fine in texture as not to allow of
the rays of light passing through it, unless two or three times doubled. I
have proved to my own satisfaction that the papers will not bear exposure
in a bag of single thickness, without browning over immediately the
developing fluid is applied.
With regard to the using of thin collodion, as recommended by Mr. Hardwick
in the last Number of the _Photographic Journal_, I am satisfied it is the
only plan of producing thoroughly good positives; and I have been in the
habit of thinning down collodion in the same manner for a long time,
finding that I produced much better pictures with about half the time of
exposure necessary for a thick collodion.