350 On the Refraction of Light. 



retort is broken,* and the acid thrown into large stone ware jars and 

 thus sold. The colcdthar is extracted from the retorts by means of 

 the scraper; it is of a dark red color, having occasionally a yellowish 

 hue, which indicates but a partial decomposition. This oxide is sent 

 back to Goslar, where it is manufactured into ochre. Those of the 

 retorts that are no longer fit for service are replaced. Twenty four 

 hours suffice for collecting the acid, and making the necessary prep- 

 arations for a new operation. The quantity of sulphate of iron em- 

 ployed in each charge, is always effloresced in the lateral chambers, 

 by a preceding heat. Thus each furnace is heated three times per 

 week, leaving the workmen at liberty the seventh day. 



The depot of this acid is at Frankfort, on the Mayne ; in Paris its 

 first cost is forty sols the pound. The sulphate of iron costs 1 thaler 

 and 12 bons groschens the quintal. The four furnaces consume at 

 one distillation seven matters of wood, giving a quantity of acid, ac- 

 cording to the manner that the distillation has been conducted, from 

 200 to 260 lbs. which is sold at 15 bons groschens the pound. The 

 colcothar sells for 12 bons groschens the 5 quintals. The cylinders 

 last a very long time ; thus the expenses for breaking are trifling. 



Art. XX. — Observations and Experiments on the phenomena de- 

 veloped by Light, in its passage through small apertures; with re- 

 marks on some of the received theories, and an investigation into 

 the cause of prismatic analysis; by Columbus C. Conwell, M. D. 



TO THE EDITOR. 



The experiments and observations contained in the following pa- 

 per, relate to the partial decomposition which hght undergoes in pass- 

 ing through small orifices, and impinging on the surfaces of bodies, 

 and to other phenomena of vision. Although the subject has long 

 exercised the ingenuity and industry of studious minds, the phenom- 

 ena appear to be inconsistent with the existing hypotheses. Without 

 recapitulating theories, or citing the authorities by which they are 

 sustained, I proceed to the statement of the following facts, happy if 

 I have augmented the number of exact observations, and thus aided 

 in any degree in advancing this interesting branch of science. 



* The lute is moistened and reserved. 



