TRANSACTIONS OF THE SECTIONS. 69 



presence of a free alkali. The powers of red prussiate of potash as a discharger or 

 bleacher of organic colouring principles have been successfully applied by Mercer*, and 

 its action as an oxidizing agent fully examined and explained by Playfair, Baudraultf, 

 Wallace;!:, and others. Its rapid action upon the colouring matter of cochineal 

 may be seen by adding a solution of the salt to cochineal dissolved in a weak ley of 

 caustic potash or soda, when the rich purple colour of the cochineal liquor will be 

 speedily discharged. 



In applying this action to testing the quality of commercial samples of cochineal, 

 certain precautions require to be strictly observed, and of these the most important 

 are, to use the solution of cochineal perfectly cold, and to finish off the process as 

 quickly as possible. 



Process. — A fair quantity of the sample being finely pulverized, 20 grains are 

 weighed out, and gently heated in a beaker with half an oinice of caustic potash 

 solution and one ormce of water. When the colouring matter is completely dissolved, 

 one ounce of cold water is added, and the mixture allowed to cool. 



An alkalimeter is made up with 5 grains of pure and dry red prussiate of potash 

 in the usual way. This solution is then dropped into the cochineal liquor till the 

 rich purple colour is discharged, and the liquor assumes a yellowish-brown tint. The 

 moment when this effect is produced may be easily ascertained by occasionally spot- 

 ting a little of the liquor upon a white slab. The number of measures consumed 

 shows the comparative richness of the sample in available colouring matter. 



In applying this method to lac-dye, the operations are the same as for cochineal, 

 except that a larger quantity of the lac must be employed, as the amount of colouring 

 matter in it is small compared with that in cochineal. 



The accuracy of this process may of course be easily vitiated by the presence or 

 addition of any substance that acts chemically upon the agent of valuation. But 

 nearly all volumetric methods of analysis are open to this objection; and hence they 

 cannot be considei-ed as intended for the use of those who have not sufficient chemical 

 knowledge to guard against such obvious sources of error. 



On the Manufacture of Iodine and other Products from Kelp. 

 By Dr. F. Feuhy, F.aS. 



In the course of his remarks. Dr. Penny stated that the results of some hundred tests 

 showed the quantities of the several ingredients found in kelp to be as follows : — In good 

 drift weed — soluble matter 75, insoluble matter 22, water 3, iodine per ton 14 lbs., potash 

 salts 7 cwt. In the inferior drift-weed, which had been adulterated with sand and 

 stones, the proportions were — soluble matter 40, insoluble matter 50, water 10, iodine 

 2 lbs., potash salts 3f cwt. In cut weed, the proportions were — soluble matter 60, 

 insoluble matter 35, water 5, iodine 2| lbs., potash salts 5^ cwt. The average produc- 

 tion from a ton of kelp was, from drift-weed kelp — iodine 12 lbs., muriate of potash 

 4| cwt. (80 per cent.), sulphate of potash 2| cwt. (55 per cent.), alkaline or fished 

 salt 2| cwt., and refuse sulphur ^ cwt. From cut-weed kelp the production was — 

 iodine 2| lbs., muriate of potash 3^ cwt. (75 per cent.), sulphate of potash 2^ cwt. (30 

 per cent.), alkaline or fished salt 31 cwt., and refuse sulphur ^ cwt. 



On the Composition and Phosphorescence of Plate- Sulphate of Potash. 



By Dr. Fred. Penny, F.C.S., Prof, of Chem., Andersonian Inst., Glasgow. 



[This paper may be referred to in Phil. Mag. Dec. 1855.] 



On a Process for obtaining Lithographs by the Photographic Process. 

 By Professor A. C. Ramsay, F.R.S. 



Prof. Ramsay described a process by which Mr. Robert M'Pherson, of Rome, had 

 succeeded in obtaining beautiful photo-lithographs, — specimens of which bad been 

 hung up in the Photographic tlxhibition in Buchanan Street. The steps of the procesi 

 are as follows: — 1. Bitumen is dissolved in sulphuric acid, and the solution is poured 

 on an ordinary lithographic stone. The asther quickly evaporates, and leaves a thin 

 coating of bitumen spread uniformly over the stone. This coating is sensitive to light, 



* Chem. Soc. iii. t Jouru. Pbarm. vii. % Quart. Joum. vol. vii. 



