VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 40Q 



to us ; and if they had any considerable motion, it would most likely have been 

 discovered, since the variations of Sirius, Arcturus, Procyon, Castor, Pollux, &c. 

 have not escaped our notice. Now, from the same principle of the motion of 

 the solar system, by which we have accounted for the apparent motion of the 

 latter stars, we may account for the apparent rest of the former. Those two 

 bright stars, a. Lyrae and a. Orionis, are placed so near the direction of the 

 assigned solar motion, that from the application of my 2d theorem, their motion 

 ought to be insensible in right ascension, and not very considerable in declina- 

 tion ; all which we find is confirmed by observation. With respect to Rigel and 

 a. Serpentarii, admitting them both as stars large enough to have shown a proper 

 motion, were their situation otherwise than it is, we find that they also should 

 be apparently at rest in right ascension ; and Rigel having southern declination, 

 and being a less considerable star than a Orionis, which shows but 11" motion 

 towards the south in 50 years, its apparent motion in declination may, on that 

 account, be also too small to become visible. 



XVIII. Some Experiments on the Ochra friabilis nigro fusca of Da Costa, 



Hist. Foss. p. 102 ; and called by the Miners of Derbyshire, Black JFadd. 



By Josiah Wedgwood, F. R. S. p. 284. 



The extraordinary circumstance of this substance taking fire on being slightly 

 mixed with linseed oil, first discovered by accident in the year 1752, at Mr. 

 Bassano's, a painter in Derby, has rendered it a subject of curiosity ; but, as it 

 is now employed in considerable quantities, and very advantageously, as an oil- 

 colour in ship and house-painting, it has a better claim to our attention. 



Many years before, Mr. W. first collected some of this earth, which basseted 

 out in a hollow way, near Winster, in Derbyshire, and he tried some experi- 

 ments on it ; but as they were not very interesting to him at that time, and 

 being occupied with other matters, he made no further use of it till Dec. 1782, 

 when a series of experiments being made at the President's house on its in- 

 flammable property when mixed with oil, at which John Walsh, Esq. and 

 several other gentlemen were present, Mr. Walsh sent him a specimen, and ex- 

 pressed his desire that Mr. W. would analyse, and make some further experi- 

 ments on this extraordinary substance. Mr. Woodward, as well as Mr. 

 Da Costa, has described this earth so minutely, that it cannot easily be mistaken ; 

 but from the following experiments it will appear, that it should not be classed 

 among the ochres not acted on by acids ; and that it may, with as great pro- 

 priety, be called manganese as ochre. 



Exper. 1. Mixed with porcelain biscuit body, it gives darker or lighter shades 

 of black and brown, as the quantity is greater or less in proportion to the body. 

 — Exper. 2. Mixed with linseed oil, in the quantity of a few penny-weights 



vol. xv. 3 G 



