ON THE NUTRITIVE VALUE OF THE FOOD OP MAN. 



1853.] 



mains on the paper, the hyposulphite of soda will dissolve some 

 portions of it, and thus injure the pictui'e. This is shown by the 

 more energetic action of the hyposulphite on the positive than on 

 the negative pictures. In the latter, b}' the action of the Gallic 

 acid, or the ]jrotosulphate of iron, the complete deoxidatiou of 

 the sih'er salt is eltccted. In the former, this is not the case 

 where the exposure to sunshine has been short, or where the copy 

 has been made by the effect of diffused daylight. 



Positive photographs which are made when the sun is shining 

 bi'illiantly, are far less liable to injuiy than such as are procured 

 by the weak and uncertain light of a wintry day, althoun-h they 

 may in both cases be brought to the same apparent degree of 

 darkness. 



2. As a general rule, it is advisable to expose the positive to 

 sunshhie longer than it is necessary to do, for the production of 

 a well-defined image. If the picture has been rendered far too 

 dark to be pl-:asant, it can be toned back, to use an artistic phrase, 

 by very weak solutions of the iodide or cyanide of pottassium. 



3. The photograph being removed from the copying frame, or 

 the camera, should bo first placed in some clean water, to which 

 a small quantity of common salt has been added. By this all 

 the free nitrate of sdver is converted into a chloride; and the 

 formation of any sulphuret of silver in the paper, by the action 

 of the nitric acid on the sulphur salt, prevented. The picture 

 should, after it has soaked for a little time, be removed and placed 

 in a solution of the hyposulphite of soda, in a flat dish — about 

 an ounce of that salt being dissolved m a quart of water — it 

 should remain in this fluid for fi\ e or ten minutes, and then be 

 removed to a vessel of peifectly clean water. 



4. It is thought by many photographers that the addition of 

 some chloride of silver to the hyposulphite of soda pre\'ents its 

 acting on the move delicate shadows of the picture. Whether 

 this is the case or not, is somewhat uncertain; but the hyposul- 

 phite solution can be used a great many times, if after using it 

 is poured back into a bottle, and kept from the air. 



5. It becomes necessary now to remove every trace of the 

 hyposulphite of soda and silver from the paper. Many persons 

 are content with soaking their pictures; but by far the best prac- 

 tice is, to place the photographs upon a flat board, incline it to 

 an angle of about 45 ° , ant! allow water slowly to fall upon and 

 flow otf from the pictures. By this means the salt is removed 

 far more rapidly than by soaking and changing the water, how- 

 soever carefully this may be done. E\'en after this the safest 

 course is, to place the photograph in some clean hot water, to 

 which a little potash has been added. This secures the removal 

 of the last trace of the hyposulphite, and it darkens again those 

 lin s of the photograph which may have been injured by chemical 

 action, as above described. 



6. By attention to these details photographs may be fixed 

 most permanently, without their undergoing any serious injury. 

 The addition of neutral chloride of gold to the hyposulphite of 

 soda bath, tends to produce a variety of purples approaching al- 

 most to black, which are of a very pleasing character. Similar 

 results may be obtained by soaking the picture in a weak solu- 

 tion of the chloride of gold, upon removing it from the fixing 

 fluids. 



7. The experience derived from the photographs displayed at 

 our late Photographic Exhibition, some of which have since been 

 presented to the Society, convinces us that sufficient care is not 

 generally given to secure the perfect permanence of a fine posi- 

 tive photograph. By the combined influence of a moist atmos- 

 phere and light, changes slowly' go on from the edges of the 



Mt 



paper spreading inwards, which eventually destroy the picture, if 

 there is the slightest trace of the hyposulphite of silver allowed 

 to remain on the paper. The taste is the best test that we can 

 apply ; and if after a picture has been well washed in several 

 perfectly clean waters, we take one coiner of it into the mouth 

 and suck out some of the water, without discovering any metallic 

 sweetness, we may be sure that our photograph will enduie as 

 long as any ordinary print. — Journal of the Societi/ of Arts, 



"On the nut'itive value ef the food of Man ondrr diffrrrnt 

 cuiiUitioiis of age and cmploynieiit." By Dr« L.. Playiair 



The great importance of an attentive consideration to the kinds 

 of fcod taken under ditt'erent circumstances becomes evident 

 when it is known that one class of substances supplies the fuel 

 that maintaius the heat of the body, and other substances supply 

 the materials that form the flesh and the bones. The lungs act 

 as a furnace, in which the process of slow combustion is always 

 going on by the absorption of oxygen from the air into the blood 

 and the exhalation of a portion of it, in combination with the 

 carbon of that fluid, in the form of carbonic acid. It was stated 

 by Dr. Play fair that the weight of oxygen absorbed by a man 

 in this manner in a year, averages 700 lbs., and that the con- 

 sumption of carbon during the process is so rapid that in the 

 course of three days all the carbon in the blood would be ex- 

 hausted, if it were i ot renewed by a supply of proper food. As 

 the temperature of the body is always the same uuder every cli- 

 mate, the inhabitants of the colder regions of the earth require a 

 larger amount of food containing carbon than those who live 

 further souti', to maintain the heat at its requisite standard. Fire 

 and warm clothing diminishing the want of heat- producing food, 

 therefore it becomes a question, in an economical point of view, 

 whether it is not cheaper as well as better to keep paupers and others 

 who are supported at the national cost, well clothed and in warm 

 rooms, and thus to supply externally by low-priced fuel a portion of 

 the animal heat that would otherwise have to be maintained by the 

 more costly fuel supplied to the stomach as heat-producing food. 

 The substances that contain the greatest amount of carbon are 

 those which best supply heat; among these sugar and rice are 

 prominent ; whilst the flesh-giving substances are those that con- 

 tain nitrogen — meat, peas, and cheese, being the most abundant 

 sources. As different kinds of sohd food produce difterent eflFects 

 in the nutriment of the body, it is requisite in a well-regulated 

 dietary that tlie proportions of flesh-giving and heat-producing 

 food should be jiiopeily adjusted, taking into consideration age, 

 employment, and climate. The regulations tor dieting sailora 

 exhibited, at one time, great ignorance of this requisite attention, 

 and in the dietary equivalents of the navy in some instances, 

 heat-producing food was substituted almost to the exclusion of 

 flesh -giving f )od, all kinds of solid nutriment being ignorantly 

 considered to operate in the same manner. Dr. Playfair noticed 

 at considerable length the difficulty in obtaining accurate statistical 

 statements of the dietaries of different classes, but he nevertheless 

 exhibited numerous diagrams, representing, by differently coloured 

 lines of various lengths, the respective quantities of food of both 

 kinds allowed to soldiei-s, sailors, paupers, and prisonere in this 

 and other countries. He pointed out strongly the facts which 

 had come to light during Mi'. Chadwick's investigations respecting 

 the rJative quantities of nutriment of agricultural labourers and 

 prisoners. From this it appeared that whilst the agricultural 

 labourer had a scanty allowance, scarcely sufficient to maintain 

 vigorous life, the suspected thief was sufficiently fed, the convict- 

 ef thief was still better treated, and when he arrived at the dig- 

 ■ nity of a transported con\'ict, he has double the allowance of the 

 hard-working labourer. Dr. Playfair mentioned a curious fact, 



