168 ANNUAL OF SCIENTIFIC DISCOVERY. 



plate, such as the chlorides of sodium, aluminum, magnesium, &c., 

 lout if there be added to their solution a salt of copper, they become 

 fit for giving an impression on the plate of silver, and producing 

 colors, those colors predominating, at the will of the operator, accord- 

 ing to the quantity of the chloride added to the salt of copper ; the 

 proportions of the salt of copper vary according to the chlorides 

 employed. This result is the more remarkable because the salts of 

 copper employed without a chloride exercise no influence, and this 

 influence varies according to the quantity of chlorine or chloride put 

 into the bath with a quantity of salt of copper remaining the same. 

 In like manner, by varying the proportions of the salt of copper, 

 with a given quantity of chlorine or chloride, remaining the 

 same, the effects can be changed ; but in this case the results 

 will be similar to those of the former ; however, it is preferable 

 to take 100 grammes of sulphate of copper, with 400 parts of 

 water, and add variable proportions of chlorine or chloride, according 

 to the color required. To obtain all the colors at once, there must 

 be taken the proportion of chlorine or chloride corresponding to the 

 yellow and green rays, and in this case there will be several colors 

 by allowing the plate to become suitably prepared in the bath ; that 

 is to say, the bath ought always to be at a temperature of 10 Centi- 

 grade, at the least, and the plate should be immersed in it for about 

 five minutes. The thickness of the coating laid on, as well as the 

 absorption of the bath, causes a difference in the effect ; it is there- 

 fore very essential to operate always in the same condition if the 

 same results are desired. (The mixture of the chloride with the salt 

 of copper should be made cold, or at least at a moderate tem- 

 perature.) 



When several colors are obtained on the plate they are much less 

 vivid than when only one predominant color is required. This is the 

 reason that it is so difficult to obtain several colors at once, of great 

 intensity, particularly with white grounds, and to copy the dark parts 

 at the same time. I mentioned that the feeblest quantity of chlorine 

 or chloride furnished yellows ; but to have indigo and very bright 

 violet, there will be no yellow. The red, alone, is always produced, 

 because this color is caused by the heat of 100 Centigrade, to which 

 the plate has been first of all exposed before any action of the light ; 

 however, with yellows, the red is very feeble. The finest reds are 

 obtained with a large quantity of chlorine or chloride, except with 

 the acid chlorides, such as those of zinc and tin, and hydrochloric 

 acid, which furnish very good results when they are mixed with a 

 salt of copper in suitable proportion ; but if in excess, there will be 

 produced only a violet color. In this case the ground of the picture 

 is very clear, and the lines very pure. With the neutral chlorides, 

 when united to a salt of copper, it happens that, if in excess, they 

 produce very bright colors, particularly the reds and those of an 

 orange tint, but the ground of the x plate is always dark ; this is the 

 case particularly with the perchloride of iron. If a mixture is made 



