250 PHILOSOPHICAL TRANSACTIONS. [anNO 1728. 



Sometimes the crystals represent two equal pentelateral pyramids joined at 

 their base. But, under whatever form these crystals shoot, they always carry 

 an exceedingly fine surface ; which, when rubbed off, cannot be renewed by 

 any art. In fig. 14, one side of the parallelopepidon is rubbed away, to show its 

 appearance after losing its natural surface. 



These crystals are of different colours, from the white, like white sugar 

 candied, to the deep black. The white crystals seem to carry a finer lustre than 

 any other, and are perfectly transparent ; so that were they found of equal size 

 with the black crystals, and of a white water, their hardness and weight, in 

 both which they exceed any other fossil, would probably make them preferable 

 to the diamond. However, as the deeper colours of these crystals seem to 

 arise from a greater proportion of iron in their composition, which they throw 

 off" in an iron slag on fusion, and which changes by proper degrees of heat into 

 a crocus, thereby changing the colour of the crystal to a brighter red ; so the 

 white tin ore is certainly to be esteemed both richest and best, as most free 

 from iron. 



These crystals seem to be the heaviest bodies the earth produces, except 

 quicksilver and real metals. Their specific gravity, is to water, as QO-l to 10 ; 

 to rock crystal in water, as 904 to 26 ; to diamond, as QO-i- to 34 ; and to pure 

 malleable tin, as found by repeated trials, as QO-I- to 78 ; from whence appears 

 tlie possibility of what some miners affirm, viz. That a cubic inch of some tin 

 ores will yield more than a cubic inch of metal. 



Having already remarked that the crystals of tin are sometimes so small as to 

 escape the eye, and so disseminated in the load, as not to make above the 800th 

 or 1000th part of the load, one would naturally imagine it an endless labour to 

 cleanse the ore from such a vast disproportion of rubbish. But the great specific 

 gravity of these crystals renders the cleaning of it less troublesome, and less ex- 

 pensive, than in any other ore whatever. It requires no more, than that the 

 whole stuff" be stamped to a fine powder, after which it is washed with a water, 

 whose force is so moderated as to wash away only the lightest parts. This 

 stamping and washing is repeated till the ore is left exceedingly clean, and yields 

 in metal from -^f to ^°4h, according as it is cleansed from the load, and as it is 

 in its own nature more or less free from iron. 



^ Method of raising some Exotic Seeds, which have been judged almost impossible 

 to be raised in Englarid. By Mr. Philip Miller,* Gardener to the Physic 

 Garden at Chelsea. N° 403, p. 485. 

 Mr. Miller here gives an account of the methods he has taken to raise the 



* Phillip Miller, author of the Gardener's Dictionary, was borij in lOyi. His father was gardener 



