Exptrnntntj and Obj'ervatioiis on the Jlliigih/lc Sn-peiitint. loi 



of laminse Is feen. This is in the direQion of the magnetic poles. The tenacity is very 

 confiderable, as it was not broken but by a violent blow on a round flint. Hardnefs be- 

 tween 6 and 7 of Kirwan ; that is to fay, it yields to the knife and fde, but greatly injures 

 thofe inftruments. Powder white, or gveenilh-white. Fracture coarfe, earthy. Frag- 

 ments rather angular. No fmell, nor adhefion to the tongue; but when t«'o pieces are 

 flruck together its fmell is the fame as is given in like circumftances by rock cryflal. Under 

 the hammer it gives fire, but rarely with the (Icel. Specific gravity 2.91. Does not im- 

 bibe water. When touched with nitrous acid, a very (light efFervefcence took place, which 

 was general and diftinft, but fcarcely vifible without the magnifier. When ftrongly urged 

 with the blow.pipe, it acquired an irregular light-brown colour, and was much lefs tenacious, 

 but fuffered no other change. Acid of barax and microcofmic fait diflblve it with little 

 efFervefcence, and very flowly. The glafs of the former is clear green, the latter clear white. 

 Its whole weight was 8238 grains ; and that of the piece broken off for experiment, 

 without the conta£t or ufe of iron, was 614 grains. It weighed 403 grains in water, and 



confequently loft 211 grains. Hence = 2.91, or the fpecific gravity to water aflumed 



as 1. 00. This differs from the refults found by Mr. Humboldt. 



The frafture was made acrofs the line of magnetical diredlion. The furfaces which had 

 been thus feparated, exhibited oppofite poles, in the fame manner as when any other na- 

 tural magnet is broken. The fmaller piece was tlien broken into many fragments, and in 

 part pulverifed. All the pieces were poffeffed of polarity. A fmall five-bar horfe-flioe magnet 

 took up the fmaller fragments, and the powder, though weakly, as they were eafily fliakcn 

 off. I could not fatisfaftorily afcertain that any part of the Rone did either attract the 

 fineft iron filings, or influence their arrangements when laid upon paper. 



A very delicate magnetic needle, i| inches long, moving on an agate focket, was fuffered 

 to difpofe itfelf in the magnetic meridian. The larger piece was prefented due eaft from 

 the centre, and then moved upon its own centre, to find the pofitions of the greateft at- 

 traction and repulfion. At tlie diftance of 18 inches from the middle of the ffone, the 

 needle was perceptibly affefted ; but its greateft deviation did not exceed ten minutes of 

 a degree. At 12 inches the deviation was one degree, and at fix inches it was about 14. 

 degrees. By application of the ftone clofe to the compafs-box, the needle was led into any 

 pofition at pleafure. 



It was found on comparifon with various pieces of natural magnet, that its directive 

 force or effe£t on the needle is much weaker than that of any of the pieces. But whether 

 a natural magnet as weak in dire£live power as this ftone would be equally inaftive 

 with regard to iron filings, does not appear from the fa£ls I have hitherto obferved. 



II. 

 All Account of /owe Experiments upon Coloured Shadows. By Lieutenant General Sir B/ilf- 

 JAMIN Thompson, Count of Rumfud, !•. R. S. In a Letter to Sir Joseph Banjc.j, 

 Bart. P. R. S.« 



SDEAR SIR, 

 I\'CE my laft letter, being employed in the profecution of my experiments upon liglit, 

 I wa« ftrutk with a very beautiful, and what to me appeared to be a new appearance. Dc- 



• From (he riiilufmliic»l Tmnliftioiis, 1794. p. 107. 



ft tout 



