74 



salt and yeast, in quiti small quantities, their influejice, both on the 

 physical nature and on the keeping qualities of the bread (so far as 

 staleness is concerned), is out of all ajjparent proportion, however, 

 to the amount present. The present writer has already remarked 

 elsewhere that : — 



" The effect of freshness can be enormously increased and sustained 

 for many days by the addition of small quantities of fat. With 

 added fats, up to 3 lbs. to the sack of flour, to doughs made by the 

 short, straight process, the colour is but little impaired, whereas 

 the crust is shorter, the crumb sweeter and more palatable, and the 

 effect of staleness is not appreciated for a much longer time than is 

 the case with bread from the simple standard mixing. The use of 

 half milk and half \\ ater, instead of all water, aa liquor has a some- 

 what similar effect, and the bread produced from this mixing recalls 

 the home-made farm loaf, which does not appear to change from 

 the original state of freshness for a week or more, if kept in a cool, 

 dry place." 



The reason is not so far to seek, yet, up to the present, no figures 

 have been obtained to demonstrate the protective influence of the 

 colloid existence of these added ingredients upon starch solutions. 

 There is little doubt, however, that they prevent the deposition of 

 starch in much the same way as they oppose the setting of cements. 



COLLOID CHEMISTRY IN PHOTOGRAPHY. 



By R. E. Slade, M.C, D.Sc, F.I.C, Director of Research, British 

 Photographic Research Association. 



Introduction. 



Most photographic processes fall into one of the two following 

 classes : — 



(1) The substance sensitive to the light is eventually turned 

 into the pigment. 



(2) The substance sensitive to light is the support of a 

 pigment already present. 



In both classes of process the support of the sensitive substance 

 or pigment is usually a dried gel, e.g., collodion, gelatin, gum. 



As an example of class (1) we will consider the ordinary commercial 

 dry plate. The sensitive film consists of very small crystals of sUver 

 bromide, sometimes containing some iodide, supported in a dried 

 gelatin gel. On exposure to light some of the grains become develop- 

 able. On development — treating with a reducing agent, e.g., alkaline 

 hydroquinone — the gelatin swells and the developer diffuses into the 

 gelatin, reaches the grains of silver bromide and reduces to metallic 

 silver those previouslj' made developable by Ught. 



As an example of class 2 we will consider the carbon process. 

 A gelatin solution containing a pigment such as finely divided carbon 

 is coated on to a suitable support, dried, sensitised by immersion 

 in a solution of ammonium bichromate and dried again. The print 



