optical Phenomenon. — Odors rendered vifihle. 43 1 



the fun will fvvell out, and join the other' fti^dpw' when the diftance becomes diminiflied to 

 a certain quantity, while the fliadow of, the body nsaieft the fun undergoes no alteration. 

 What is the nature of the inflection of light which occafions this appearance ? 



I am. Sir, yours, &c. 

 . A. Y. 



.*;j* The fact mentioned in the lafl paragraph of this letter has been flated and explained 

 elfewhere, though I cannot now recolleft where I have feen it. It does not depend upon 

 infleilion, but upon the different widths of the penumbrje. When the penumbr2e firll inter- 

 fere, a faint lenticular fhade is produced, which is nearer the fliadow of the moft remote 

 body than that of the other ; and as foon as the remoter body itfelf comes in contaft with 

 the bounding ray of the greater penumbra, it begins totally to intercept the fun's light on the 

 fide next its own fliadow. The time employed in pafling from one extreme ray of the 

 penumbra to the other will be lefs the nearer the bodies are to each other, becaufe the diftance 

 of thofe rays is lefs the nearer the vertex. The fliadow of the remoter body will confequently 

 run over the penumbra with greater fpeed than that of its former progrefs; and if any part be 

 convex, it will advance beyond the others. 



To Mr. Nicholson. 

 SIR, 



HAVING obferved in the 4th Number of your Journal *, and likewife in the Annales 

 de Chimie, an extraft of Mr. Prevofl;'s Memoir on the means of rendering vifible the 

 emanations of odorant bodies ■■, I take the liberty of communicating an obfervation which 

 occurred to me a few years ago, while making experiments with different fubftances ex- 

 pofed to the oxigenated muriatic acid-gas. That which afforded the curious phenomenon 

 to ^yhich I allude, was fome highly-reJlified animal oil. This matter, immediately on 

 being expofed to the gas (which was in a very dry flate),'was feen to emit a copious fl;eara, 

 the particles of which rifing to the height of about four inches above the fmall phial in 

 which the oil was contained, were then obferved gradually to defcend, forming a very cu- 

 rious and pleafing appearance. — Although ftruck at the time with the Angularity of this 

 phenomenon, and endeavouring to account for the formation of the vapour by a fuppofed 

 union of the hydrogen of the emanating matter with the fupeir-abundant oxigen of the gas ; 

 yet I omitted at the time, and have fince negle£led, to endeavour to verify my conjeftures 

 by further trials. — The circumftance, however, revived in my recolledtion on reading 

 your Journal ; and conceiving it to be an experiment that affords a nearer approach to the 

 MoXxitt perceptibility of an odorant vapour, than thofe flated in the Memoir of Mr. Prevoft, 

 I impart it for your further inveftigation, if thought worthy your attention, and am 

 ' ' Your obliged reader, 



JVov. 24, 1797. W.HOWARD. 



N. B. The oil was the only odorant fubflance tried : perhaps many others might afl'ord 

 a fimilar or varied appearance. 



•;j* The philofophical confideration of odorant bodies is fomcwhat obfcurcd by the old 

 method of gcncralifihg, or referring the properties of bodies to fomc dillindt principle or 



• Pag» »53. See alfo on this fuhjeft, p. «05.' N. 



tiling 



