1853.] 



SCIENTIFIC INTELLIGENCE. 



21 



This was a difficulty. But after some researches I found a liquid 

 which etches steel perfectly -well, and at the same time is free from the 

 inconvenient property of penetrating the gelatine film. This liquid is 

 the bichloride of platiua. In order, however, to use it successfully, it 

 must be mixed with a certain quantity of water, neither more nor less, 

 (I mean to any material extent,) otherwise its action becomes irregular. 

 The best way is, to make a perfectly saturated solution, and then add 

 to it one-Iourth of its bulk of water. Then correcting this by a few 

 trials, a solution of proper strength is finally obtained. Supposing 

 then, that we have prepared such a solution, the operation of etching 

 the plate is performed as follows:— The plate is laid on a table, an i a 

 small quantity of the bichloride being poured upon it, it is to be 

 rapidly diffused and spread over the whole plate with a camel hair 

 brush. Not much liquid is poured on, because its opacity would pre- 

 vent the operator from distinguishing the effect produced by it on the 

 metal. For this reason, it is hardly necessa.ry to make a wall of wax 

 round the plate ; that is, if the portions to be etched are confined to the 

 central part of the plate, and do not approach very near to the edge. 

 The effect of the liquid upon the plate is not at first visible, since it 

 disengages no gas ; but a*ter the lapse of a minute or two, the white 

 photographic image begins to darken, and soon becomes black in every 

 part. When this change is complete, the image often looks very beau- 

 tiful, though quite altered from what it was before. The operator 

 should carefully watch the image until he thinks that it is finished, or 

 not likely to be further improved or developed by continuing the pro- 

 cess any longer. He then inclines the plate gently, and pours off the 

 liquid by one comer of the plate. The plate is then dried with blot- 

 ting paper, aud then a stream of salt water, which is better than fresh 

 water for this purpose, is poured over the plate, which removes all 

 traces of tie etching liquid. The plate is then rubbed with a wet 

 spcnge or linen cloth, which in a short time detaches ;iud removes the 

 film of gelatine, and discloses the etching that has been effected. 

 When the object is not of a nature to be applied directly to the surface 

 of the plate, the most obvious method of proceeding is, of course, to 

 place the prepared plate in the focus of a camera, and to direct the 

 camera to the object. But m consequence of the low degree of sensi- 

 tiveness of bichromate of potash, this would take, generally speaking, 

 too long a time to accomplish. The better way in practice, therefore, 

 is, to take a negative photograph of the object on paper with a camera, 

 aud from this to obtain a positive copy either on glass or paper, which 

 should be very uniform in texture, and moderately transparent. Then 

 this positive copy is placed on the plate in a photographic copying 

 frame, and being placed for a few minutes in the sun, it impresses the 

 plate with a photographic image; which image, etched as above de- 

 scribed, and printed off upon paper, will finally give a positive repre- 

 sentation of the object. If the object depicted upon the plate by the 

 sun's rays is broad aud uniform, for instance, the opaque leaf of a plant, 

 then, of course, the etching is uniform also. When this is printed off, 

 it produces an effect which is not always satisfactory. I will therefore 

 now explain a modification of the process which destroys this uniform- 

 ity, and which in many cases produces a great improvement in tho 

 general effect. 



For this purpose I must remark, in the first place, that if a piece of 

 black gauze or crape is the object selected for representation, it produ- 

 ces an engraving of itself which is marvellously accurate. But when 

 two folds of the gause are laid across each other obliquely, then the 

 resulting engraving requires a lens, in order to separate from each 

 other and distinguish clearly the lines belonging to the two portions 

 of the gauze. Now, if this engraving is printed off, the result offers to 

 an eye at a moderate distance the appearance of an uuifom shading. 

 Now, I avail myself of this circumstance, to modify my original pro- 

 cess as follows; suppose the object to be the opaque leaf of a plant, of 

 irregular outline ; first, I cover the prepared plate with two oblique 

 folds of black crape or gauze, aud place it in the sunshine for two or 

 three minutes. The effect of this is, to cover the plate with a compli- 

 cated image of lines passing in all directions. Then the leaf is substi- 

 tuted for the crape, and the plate is leplaced in the sunshine for two 

 or three minutes more. The leaf being then removed from the plate, 

 it will be seen that the sun has obliterated all the lines that were visi- 

 ble on the parts of the plate exterior to the leaf, converting all those 

 parts to a uniform brown. But the image of the leaf itself is still 

 covered with a network of innumerable lines. Now, let this be etched 

 in the way already described, a"ud let the resulting etching be printed 

 off. The result is an engraving of the leaf, which when beheld by the 

 eye at a certain distance appears uniformly shaded, but when examined 

 closely is found to be covered with lines very much resembling those 

 produced by an engraver's tool, so much so that even a practical en- 

 graver would probably be deceived by the appearance. This crape 

 arrangement 1 call a photographic veil: and as 1 think it likely that the 

 idea will prove useful, I will make a few more remarks upon it. It is 

 clear that an arrangement composed of two thicknesses of ordinary 

 crape or gauze is but a rude attempt at a photographic veil. To real- 



ize the practical utility that may result from the idea, supposing it to 

 be borne out by further experience, it would be proper to fabricate a 

 much finer material, and to employ five or six thicknesses of it, or else 

 to cover a sheet of glass in any convenient manner with an innumera- 

 ble quantity of fine lines, or else with dots aud specks, which must be 

 ooaque aud distinct from each other. The result of practically em- 

 ploying such a method, supposing always that it answers in practice, 

 as I think it probably will, would be an etching apparently uniform, 

 but really consisting of separate small portions, iu consequence of which 

 it would hold the ink much better, and other obvious advantages 

 would also be obtained. Another mode of accomplishing the same 

 object is to cover the plate originally with au aquatint ground. But 

 then a fresh one would be required for every plate, whereas a single 

 veil would serve fo, any number of plates in succession. Experience 

 alone can decide between these different methods. When the etching 

 is finished, the plate should be very soon coated with wax to protect 

 it. A few hours' exposure to the atmospheric air rusts aud destroys 

 the etchings when newly made, although it does not do so afterwards. 

 The oxidation only attacks the lines of the etching, the rest of the plate 

 sustaining no injury, if the air is tolerably dry. 



Having thus described the method of producing the photographic 

 etchings, it would, I think, extend this letterto too great a length were 

 I to add any remarks upon the theory of the process, whicli will better 

 be deferred to auother opportunity. — Athceneum. 



Lacock Abbey, April 25tt. 



Deodorizing Properties of Coffee. — The London Medical Gazette 

 gives the result of numerous experiments witli roasted coffee, proving 

 that it is the most powerful means, not only of rendering animal and 

 vegetable effluvia innocuous, but of actually destroying them. A room 

 iu which meat in au advanced degree of decomposition had been kept 

 for some time, was instantly deprived of all smell, on an open coffee 

 roaster, being carried through it containing a pound of coffee newly 

 roasted. In another room exposing to the effluvium occasioned by the 

 clearing out of a cess pit, so that sulphuretted hydrogen and ammonia 

 iu great quantities could be chemically detected, the stench was com- 

 pletely removed within half a minute, on the employment of three 

 ounces of fresh roasted coffee, whilst the other parts of the house were 

 permanently cleared of the same smell by being simply traversed with 

 the coffee roaster, although the cleansing of the cess pit continued 

 several hours after. 



The best mode of using the coffee as a disinfectant is to dry the raw 

 bean, pound it in a mortar, and then roast the powder on a moderately 

 heated iron plate until it assumes a dark brown tint, when it is fit for 

 use. Then sprinkle it in sinks or cess pools, or lay it on a plate in the 

 room which you wish to have purified. Coffee acid or coffee oil acts 

 more readily in minute quantities. 



canadian exports ok wheat. 



Year Wheat. 



Bushels. 



1838 296,020 



1839 249,471 



1840 1,739,119 



1841 2,313,836 



1842 1,678,102 



1843 1,193,918 



1844 2,350,018 



1845 2,597,392 



1846 3,312,757 



1847 3,883,156 



1848 2,248,016 



1849 3.645,320 



1850 4,547.224 



1851 4,275,896 



1852 5,496,718 



It appears by the above statement that our exports of wheat in 1 852 

 were about eighteen times as great as they were in 1838. They have 

 doubled four times in fifteen years, or more than once in every four 

 years for thi last fifteen years. They are now one-half as much as 

 the exports of wheat from the United States ; and at the present ratio 

 of increase— doubling in every four years — our exports of wheat will, in 

 1856, be equal to those of the United Slates. — Leader 



Sir Joiix Fra^kliu. — To those whose knowledge is obtained, and 

 whose judgment is formed, at the fire side, this may indeed appear to 

 be a wild and hopeless expedition; but those whose practical know- 

 ledge is derived from exploration, scientific research, and hard experi- . 

 euce in those regions towards whicli our course is now directed, have 

 formed a far different opinion, and their acquisition of knowledge con- 

 stitutes them the best judges, for in their belief the probability amounts 

 to all but certainty, that either Sir John Franklin, or at least the greater 

 part of his brave band, and most likely all of them, are still alive, and 



