546 



NATURE 



{Sept. 19, 1878 



through red media and then printing positives from such, 

 and finally obtaining red, green, and blue prints and 

 superimposing them, is not a step in a scientific direc- 

 tion, since it is utterly impossible to secure monochromatic 

 colours which are pure enough to give the truth of nature. 

 Such efforts, though they may be commercially valuable, 

 yet are not to be followed with too much zeal by scientific 

 photographers. 



We may now axiomise the results we have indicated : — 



1. That the undeveloped photographic image on a 

 silver compound is formed by the reduction of that com- 

 pound. 



2. That the compound may exist in two (or three) 

 molecular groupings. 



3. That the compound can only be sensitive to the rays 

 which it absorbs. 



4. That the reduced silver compound may be rendered 

 incapable of development by combining with oxygen. 



5. That light of every refrangibility may cause an 

 acceleration of oxidation provided the compound acted 

 on absorbs such light. 



6. That the oxidation of the compound reduced by any 

 particular ray may be as rapid as the reduction, and thus 

 to give a false idea of their limit of sensitiveness [to the 

 spectrum. 



7. That the oxidation of the reduced silver compound 

 may account for the phenomenon of photographs in 

 natural colours hitherto produced. 



8. That in all probability the action of dyes on silver 

 bromide is a secondary one. 



W. DE WiVELESLIE ABNEY 



GELATIN AS A FOOD-PRESERVER 



"D EMOVAL of water and exclusion of air are amongst 

 J^ the most effective conditions for the preservation of 

 animal and vegetable foods. If you coat an egg with 

 collodion you may keep it a year, and yet will find it per- 

 fectly sound at the last. By dipping a mutton-chop in 

 melted paraffin, putrefaction will be prevented. But in 

 both these examples of preservative processes, dependent 

 upon the exclusion of air, you make use of materials 

 which are costly and uneatable. There are analogous 

 drawbacks to all similar plans for preventing injurious 

 changes in articles of food. The tinning method, and the 

 method of simple desiccation in warm dry air, are satis- 

 factory in their results ; but the range of alimentary sub- 

 stances amenable to such treatment is not very extensive. 

 In Dr. Campbell Morfit's new "Gelatin Process" we 

 seem to see several points of superiority over most of the 

 older plans for attaining the same end. It is true that 

 chemists have not been in the habit of looking upon 

 gelatin (or indeed any other similar complex nitrogenous 

 body) as likely to prevent or arrest decay. On the con- 

 trary, few solutions afford a more suitable nidus for the 

 development of fungoid germs than a liquid containing 

 gelatin. But the experience of a good many months 

 tends to show that food-preparations containing gelatin, 

 if once dried so as not to contain more than 10 or 12 per 

 cent, of moisture, do not become mouldy even when ex- 

 posed to warm and moist air. A large number of Dr. 

 Morfit's experimental mixtures have been so exposed for 

 some weeks, lying on my office table : yet they have not 

 suffered any decided deterioration. They comprise many 

 perishable foods, such as cabbage, tomato, milk, and 

 meat. Though not of equal merit as specimens of the 

 gelatin process, all are edible, and some positively pala- 

 table. Further experiment will doubtless enable the 

 inventor to improve his process by modifying it still fur- 

 ther, so as to suit a greater variety of vegetable and 

 animal foods. 



Perhaps the best way of explaining the nature of Dr. 

 Morfit's invention will be to take as an illustrative example 

 the case of milk. The mere drying-up of milk has been 

 tried with but moderate success — the resulting powder 



becoming quickly rancid on exposure to the air. The 

 preserved or condensed milk now in such extensive use is 

 in many respects a satisfactory and convenient prepara- 

 tion, but it is mawkishly sweet, containing more than one- 

 fourth its weight of added cane sugar. Moreover, in 

 consequence of this addition, the proportion of nitrogenous 

 or flesh-forming substances in it has been seriously 

 lowered. Now the substitution of gelatin for cane sugar 

 in preserving milk meets both these objections to ordinary 

 condensed milk. The milk preserves its natural and 

 moderate degree of sweetness, while the gelatin, even if 

 its own value as a nitrogenous nutrient be not considered, 

 certainly does not lower the proportion of flesh- formers ta 

 heat-givers in the product. 



In order to apply his process to the preservation [of 

 milk. Dr. Morfit directs us to dissolve i lb. of gelatin in 

 I gallon of milk at a temperature of 130° to 140° Fahr.,. 

 and then to allow the solution to set into a jelly ; this is 

 then cut into slices and dried. By employing the product 

 of this first operation in lieu of fresh gelatin, for gelatinis- 

 ing a second gallon of milk, a jelly is obtained in which 

 the milk-solids are just doubled in amount. As a gallon 

 of milk contains about 6,400 grains of these solid nutri- 

 ents, casein, milk-sugar, milk-fat, and phosphates, their 

 ratio to the gelatin will become as 12,800 to 7,000 after 

 the second operation just described. If then the dried 

 milk-jujube^ as we may call it, be again and again em- 

 ployed with successive quantities of milk, a limit is 

 reached, when the i lb. of gelatin has been incorporated 

 with ten gallons. At this stage the mixture will contain 

 no more than one part of gelatin to ten parts of the nutri- 

 tive matters of milk — a proportion of added preservative 

 material which contrasts very favourably with the 25 ta 

 28 per cent, of sugar found in ordinary preserved milk. 

 If the I lb. of gelatin required could be at once dissolved 

 in the whole eight or ten gallons of milk, the process would 

 be simplified and cheapened, but gelatinisation, an essen- 

 tial part of the method, could not then be secured. For 

 it is the gradual drying up of the slabs of jelly, witlv 

 which the animal and vegetable food-materials have been 

 uniformly incorporated, that leaves every particle of 

 changeable substance with an adequate protective coating 

 of gelatin. 



One at least of Dr. Morfit's preparations has become 

 an article of commerce. He dissolves gelatin in lime- 

 juice at a gentle heat, and after removing much of the 

 water and adding sugar, incorporates the mixture with the 

 powder of navy-biscuit. Pressed in moulds and carefully 

 dried, a granular acidulous and agreeable biscuit is pro- 

 duced, which should combine a considerable alimentary 

 value with the anti-scorbutic properties of lime-juice. On 

 analysing the lime-juice jujube, the basis of these biscuits, 

 I find about 8 per cent, of water, 8 of gelatine, 5 of free 

 citric acid, much sugar, and less than i (07) per cent, 

 of mineral matter or ash. This proportion of gelatin is 

 rather high when compared with the free citric acid, the 

 characteristic ingredient of lime-juice ; but the sample 

 analysed was made in April, 1877, and may not represent 

 the exact composition of the recent product. And it be- 

 comes a question, whether for travellers' use, it would not 

 be advisable in this preparation to neutralise a little of 

 the acidity of the lime-jriice with potash, rather than to 

 mask its presence by an excessive quantity of sugar. 

 Pure lime-juice itself contains very little potash and phos- 

 phoric acid or other mineral matter ; but that fact affords 

 no argument against the introduction of small quantities 

 of these compounds into such a preparation as that now 

 under consideration. 



It would be impossible to discuss in detail the applica- 

 bility of the gelatin process to the preservation and con- 

 centration, in an uninjured, compact, and available form, 

 of fruits, of meat, of cheese, &c., &c. But it may be 

 safely affirmed that Dr. Morfit's invention has already 

 been successfully applied in several directions, and that 

 it is full of promise for the future. A. H. CHURCH 



