336 Mr, Exleys New Demoiistration 



ammonia and potash, which are isomorphous with the cor- 

 responding hypermanganates. The primary form is a right 

 rhombic prism. The crystal of the hypermanganate of 

 silver is an acute rhombic prism. The form of the hyper- 

 manganate of barytes is completely similar to the anhydrous 

 sulphate or seleniate of soda. 



The same correspondence which exists between the cry- 

 stalline shape of the hyperchlorate and hypermanganate of 

 potash and ammonia, and that of the sulphates of barytes, 

 strontian, and lead, is to be found also between the crystal 

 of the hypermanganate of barytes and that of the sulphate 

 of soda or silver. It would seem that the law of this cir- 

 cumstance, of which I have already given several examples 

 at different times, lies very deep, and that its discovery will, 

 perhaps, enable us likewise to calculate the crystalline form 

 of compound substances from that of their elementary 

 constituents. 



The isomorphism of the compounds of hypermanganic 

 and hyperchloric acids is an important fact with respect to 

 the connexion between the crystalline form and chemical 

 composition of substances, because it enables us to compare 

 the greater part of the metals with several simple gaseous 

 substances. Manganese in its lowest degree of oxidation 

 being isomorphous with lime, oxide of copper, protoxide of 

 iron, &c. The sesquioxide of manganese being isomorphous 

 with the peroxide of iron, the oxide of chromium, and 

 alumina; manganic acid being isomorphous with chromic 

 acid, sulphuric acid, and selenic acid ; and hypermanganic 

 acid being isomorphous with hyperchloric acid, we may 

 compare the above mentioned metals, sulphur and selenium, 

 with oxygen, iodine, bromine, and chlorine. 



Article IV. 



New Demonstration of the Law of Mariotte, with Corrections 

 of a former 'pa'per . By Thomas Exley, A. M. 



{To Br. R, B, Thomson.) 



Dear Sir, — I find there is a fault in one of the steps of 

 the demonstration of the 2nd prop, relating to the com- 

 position of the forces, which I did not perceive till it was 

 particularly pointed out to me by my friend, Mr. Horner, 

 of Bath : I, therefore, wish to substitute another proof; I 



