J. L. S.
Deepening Collodion Negatives.—I have lately been trying a method of deepening collodion negatives, so as to render instantaneous impressions capable of being printed from, which I have found to answer admirably;
and although it is but a slight modification of Mr. Lyte's process described in "N. & Q.," it is a very important one, and will be found to produce far better results. The picture having been developed in the usual way, with a solution of pyrogallic acid, is whitened by means of Mr. Archer's solution of bichloride of mercury. The plate is then washed with water and a solution of iodide of cadmium poured on. This converts the white chloride of mercury, which constitutes the picture, into the yellow iodide, in the same manner as the solution of iodide of potassium recommended by Mr. Lyte; but is much to be preferred, as it produces a more uniform deposit. The solution of iodide of potassium dissolves the iodide of mercury as soon as it is formed, and therefore cannot be left on the plate until the decomposition of the chloride is complete, without injury resulting to the picture, as the half-tones are thereby lost, and those parts over which the solution first flows become bleached before the other parts have attained their highest tone; whereas the solution of iodide of cadmium may be allowed to remain for any length of time on the plate, without any fear of its injuring the negative.
J. Leachman.
Caution to Photographers.—About six months since, I procured some gun cotton from a chemist which appeared very good, being quite soluble, and the collodion produced by it was excellent. That which I did not use I placed in what I believed to be a clean dry-stopped bottle, and put the bottle in a dark cupboard. I was much surprised the other day, upon going to the cupboard, to find the stopper blown out, and the cotton giving out dense red fumes of nitrous acid. It appears to me to be almost upon the point of combustion, and I have, accordingly, placed it under a bell-glass in a porcelain dish to watch the result. I feel satisfied, however, that there is some risk, and, as it may often be near ether, spirits of wine, or other inflammable chemicals, that caution is necessary not only in preserving it at home, but especially in its transmission abroad, which is now done to some extent.
An Amateur.
Replies to Minor Queries.
Artesian Wells (Vol. ix., p.222.).—Wells are often so called without just pretence to a similarity with those in Artois, whence this name is derived. There are some natural springs in the northern slope of the chalk in Lincolnshire, near the Humber, called blow-wells, which may be considered naturally Artesian. The particular character by which an Artesian well may be known is, that the water, if admitted into a tube, will rise above the level of the ground in its immediate vicinity up to the level of its sources in the basin of the district; this basin being usually gravel, lying betwixt two strata impervious to water, formed the surrounding hills, and extending often over many miles of the earth's surface. If we conceive the figure of a large bowl, inclosing a somewhat smaller one, the interstice being filled with gravel, and the rain falling on the earth being collected within such interstice, then this interstice being tapped by boring a well, the water will rise up from the well to the same height as it stands in the interstice, or rim of the natural basin. Such is an Artesian well. Supposing this huge mineral double bowl to be broken by a geological fault, the same hydrostatic principle will act similarly.
The question of preferable put by Stylites must be governed by the cui bono. Universal adoption is forbidden, first, by the absence of a gravelly stratum betwixt two strata impervious to water; and secondly, by the excessive expense of boring to such great depths. Where expense is not in excess of the object to be attained, and where the district is geologically favourable, the Artesian wells are preferable to common ones derived from natural tanks or water caverns, first, for the superabundant supply; secondly, for the height to which the water naturally rises above the ground; and thirdly, because boring Artesian wells, properly so called, does not rob a neighbour's well for your own benefit, afterwards to be lost when any neighbour chooses to dig a little deeper than you. This is a matter with which London brewers are familiar.