246 On the casting of the Specula Jbr Reflecting Telescopes, 



action must be exerted on the flow of the blood, the passive 

 condition of the tissues and diminished capacity for oxidation 

 restrain the flow from the arteries, and there being now less 

 pressure on the contents of the veins, engorgement of those 

 vessels is the result, and this condition of things is what a 

 physician designates as congestion. 



In this manner, if we admit the existence of allotropism in 

 organic atoms, we can give a very clear explanation of the 

 condition of the circulation in the pathological states of in- 

 flammation and congestion, and also of the peculiarities which 

 in those states belong to the constitution of the urine. 



University of New York, 

 Feb. 17, 1849. 



XXXIV. On the discovery of the Chilling Process in the casting 

 of the Specula for Reflecting Telescopes, Sj-c. By Professor 

 Potter, A.M.^ F.C.P.S., late Fellow of Queen's College, 

 Cambridge. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I PERCEIVE at page 143 of the February Number, that 

 in an abstract of a paper by Mr. Lassqll read at the As- 

 tronomical Society on the 8th of December IS^S, there is the 

 following passage : — " The mode of casting the large speculum 

 which I employed involved the principle, discovered, I believe, 

 and first published, by Lord Rosse, of casting the speculum 

 on what is technically called a chill, i. e. an iron base, slightly 

 warmed, which causes the speculum to cool upwards in hori- 

 zontal strata." 



Your readers will find in the fourth volume of the new 

 series of the Edinburgh Journal of Science, namely for 1831, 

 that in a paper on improvements in the casting, working, &c. 

 of Specula for Reflecting Telescopes, I had discovered and 

 there published {page 18) the improvement in speculum metal 

 by casting upon a chilling surface. 



All known substances which were affected in different man- 

 ners by rapid and slow cooling, after being heated, had up to 

 that time indicated that it was a general law, that sudden 

 cooling induced the property of brittleness or loss of tenacity, 

 cracking, and frequent failing into fragments of the substance 

 suddenly cooled ; and that the opposite procedure of very 

 slow and gradual cooling, which was generally called anneal- 

 ing, induced toughness and tenacity in the substance. Glass, 

 many minerals, and steel, were known to be subject to this 

 law. 



