VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 2Q I 



were less completely formed before. One of them he subjoined to this 

 paper. The facts taken from M. Macquer are marked wjth an m ; those with 

 the * are the consequence of his own experiments. 



In order to save much repetition in future, it may not be amiss to mention, 

 once for all, a few particulars in the conduct of these processes. 1st. By water, 

 is always meant water distilled in glass vessels, or by means of a large tin refri- 

 geratory in Mr. Irwin's method. 2d. Only glass or china vessels are used in the 

 liquid processes. 3d. By a mortar he means those excellent ones made by 

 Mr. Wedgewood; or as will be specified at the time, a steel mortar tempered so 

 hard that it will bear the grinding of enamel in it without discolouration. 



4th. Filtres are never employed, it being found impossible to get the quantities 

 accurate where they are used. The powdery parts are allowed to subside till the 

 supernatant liquor becomes clear. This sometimes requires days or weeks ; but 

 he was ignorant of a better method. By giving the vessels a circular motion 

 round their axes, he could greatly facilitate the subsiding of the solid contents. 

 If the separating vessels are made like a common tart-dish, with a spreading 

 border, the liquors may be poured off very near, without disturbing the sedi 

 ments. 5th. Phlogisticated alkali, means the vegetable fixed alkali prepared by 

 the deflagration of nitre and crystals of tartar dissolved in water, and boiled with 

 Prussian blue in such quantity, that it will not any longer precipitate an earth 

 from an acid. 



Rowley-Rag. 



The stone which is the subject of the following experiments forms a range of 

 hills in the southern part of Staffordshire. The lime-stone rocks at Dudley 

 bed up against it, and the coal comes up to the surface against the lime- 

 stone. The highest part of the hills is near the village of Rowley. The 

 summit has a craggy, broken appearance, and the fields on each side to a con- 

 siderable distance are scattered over with large fragments of the rock, many of 

 which are sunk in the ground. In a quarry near Dudley, where a pretty large 

 opening has been made in order to get materials for mending the roads, the rock 

 appears to be composed of masses of irregular rhomboidal figures : some of these 

 masses inclose rounded pebbles of the same materials. At the distance of 4, 5, 

 or 6 miles from the hills, as at Bilston, Willenhall, and Wednesbury, the rag- 

 stone is frequently found some feet below the surface in rhomboidal pieces, form- 

 ing an horizontal bed of no great depth, and seldom of more than a few yards 

 extent. Over the whole of this tract of country it is used to mend the roads, 

 and lately has been carried to Birmingham to pave the streets. Some people sell 

 it in powder, as a substitute for emery in cutting and polishing. 



More obvious properties. — Its appearance dark grey, with numerous minute 



P P 2 



