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APRIL 17, 1885. J THE PHOTOGRAPHIC NEWS. 245 that of Cambridge, have used the method (as 1 also did, to illustrate a paper read at the York meeting of the British Association) to elicit the typical characteristics of the skulls of different races. It is, however, in ethnological inquiries, and in studies of hereditary and family charac teristics, that I think the process would be most generally interesting. It must be borne in mind by those who attempt to practise it, that off-hand methods will not avail. The adjustments must be made with judgment and extreme care to produce good effects. The difference between a very carefully-made composite, and one that has been com bined with only moderate care, is great. Explanation of Plate. In the plate published this week the composites are on the left, their respective components on the right. E is the composite of the five portraits marked with small e; oppo site it, and similarly, F is the composite ofthef’s opposite it. G is a Co-composite of E and F reversed in position, and thus represents all the ten components on the right. II is a composite of five other older faces, which are omitted for want of room. POSITIVES ON GELATINO-CHLORIDE OF SILVER. BY W. M. ASHMAN AND It. OrFOED. THIRD Article. In the previous article, we directed attention mainly towards a clear understanding of the simple acid chloride emulsion, and its uses for certain purposes. those who found an opportunity to follow us by actual experiment, have doubtless felt ere this that their exer tions were rewarded ; we should be glad to know that such was the case, since it would bear independent testimony as to the value of our remarks, besides inciting others to test some of the capabilities of the process. Upon the present occasion, we intend showing how a compound emulsion may be successfully prepared from materials to be found in every laboratory. After numerous experiments with the various salts enumerated in our last article, together with others not included in that series, we have decided in favour of two compound emulsions as being satisfactory. In each case the proportions of three-fourths of a chloride emulsion to one-fourth of an organic or other emulsion, was found to yield the best result. Mixed emulsions being always slightly washed, no very particular care need be exercised with regard to an excess of haloid, the washing waters readily adjusting the balance. As before suggested for the plain chloride emulsion, we prefer employing two chloride salts, and thus avoid crystallization or deliquescence in the finished emulsion, as instanced by the following formula : — Gelatine = 7 grms. diss, in 70 c. c. of dist. water. Ammo chloride 124 „ 2 90 Potass. , 88 „ S ” ” Silver nitrate 6 „ 20 » „ Here we have a combination by means of which the two extremes are avoided. We prefer making the silver decidedly acid, before mixing with the chlorized gelatine, and for this purpose make an addition of about ten per cent, of citric acid. The solution should be effected, and the temperature slightly raised in each case before mixing takes place. If the quantities above given be employed, the colour test before mentioned will not indicate the presence of un combined silver. To mix with the above, we take either a citrate or an oxalate emulsion ; the former produces a film readily toned in the ordinary gold toning bath, but possessing indifferent keeping qualities; the latter necessi tates a special gold bath of the thio-cyanate form, and has the property of retaining its whiteness for a long period. Perhaps we should rather say that it is quite possible to tone an oxalate prepared emulsion film with the ordinary borax bath, provided it be treated either before or after toning with a dilute solution of ammonium thio-cyanate. In the first instance the following formula will form a proper proportion to blend with the above :— Gelatine 2' gram. dis. in 30 c.c.s of dist. water Amm. cit. 1’8 „ „ 6 „ „ „ Silver nitrate 2 ,, „ Lin Acid cit. -3 „ „ J ” ” The presence of citric acid helps to prevent the formation of a compound of gelatine and silver ; it also hinders coagulation of the gelatine by the ammonium salt—an event which always takes place sooner or later with gela tine when citrate and some other salts are present, unless there has been so much washing that the resulting pictures are poor and thin. The colour test applied to this formula will be somewhat disappointing, notwithstanding the excess of silver salt used ; nor is it really possible, when dealing with such organic compounds, to define the chemical changes that take place. On the application of potassium bichromate solution to a drop of this emulsion, no change may be apparent for a few seconds, but after a minute or so the red chromate of silver almost invariably appears. The argentuous citrate is a soluble salt, but, as stated by both Roscoe and Fownes, the argentic citrate is insoluble. Besides this, there are most probably two or three salts, acid and neutral, and we are inclined to think that it is either the accidental formation of an argentous or a neutral argentic soluble salt that the colour test really in dicates. This most probably only takes place upon the introduction of the chromic acid, because the addition of a chloride salt to the emulsion will not prevent the exhibition of free silver upon testing. Another argument in favour of this theory is found in the suggestion of a chemical authority to use argentous citrate as a means of getting argentous chloride by the addition of sodium-chloride. Any attempt to meet the case by successive additions of ammonium citrate will end in the apparent increase of free silver, and the ultimate separation of the gelatine from the precipitate and coagulation thereof. Thus it appears evident that the citrate group are some what unstable in their behaviour when in combination with silver and its haloids, bringing about a train of com plications which can only be unravelled by further ex periment when the precise reactions may be chemically defined. Having the mixed chloride and the citrate emulsions prepared, a mixture may be effected by adding one part of the latter to three parts of the former, and melting—if solidified—at as low a temperature as practicable. When the citrate emulsion is thoroughly incorporated with the chloride they should be rapidly set, thus reducing the evil effects of temperature to a minimum. Although three parts of chloride to one of citrate emulsion has been mentioned, the proportions must always remain a matter of individual taste, since printing colour is thereby affected ; thus, the larger the proportion of citrate employed, the redder or browner will the printing colour be ; and the less the proportion of citrate used, so will the colour tend towards the purples. When four-fifths of a chloride emulsion to one of citrate is employed, a rich violet print ing colour is obtained. After an interval has elapsed sufficiently long to ensure a firm jelly, the compound emulsion may be broken up by squeezing it through coarse wire mesh or mosquito netting into a vessel of cold water, where the thready emulsion may remain five minutes ; the whole should then be thrown on a filter or fine hair sieve, and a second washing in clean water be given, the duration of each washing being five minutes only. As a general rule, the more the washing is prolonged, the better the emulsion will keep, but the re sulting image will be less vigorous ; for this reason we
