﻿332 Prof. R. Ball's Lecture Experiments to 



from which he also deduced the same formula, 



9ROSi0 2 + Fe 2 3 , 3Si0 2 , 

 being 



9^FeO Ui0 2 + Fe 2 3 Si0 2 . 

 VVMnOj 



The minerals are also perfectly identical in constitution, the 

 only difference being that the percentage of manganese in the 

 English Babingtonite is considerably less than in the specimen 

 from Arendal analyzed by Rammelsberg, which also contained 

 no alumina, The Arendal Babingtonite, however, appears to 

 vary considerably in composition in different specimens ; for its 

 analysis by Arppe* afforded him only 1*8 per cent, protoxide of 

 manganese along with 0*3 per cent, alumina, whilst another 

 analysis by Thomson t showed as much as 6"48 per cent, alumina 

 and 10*16 per cent, protoxide of manganese. As, however, 

 neither of these chemists had determined the state of oxidation 

 of the iron in the mineral, the numerical results of their analyses 

 are useless for the purpose of determining the constitution or 

 formula of the mineral itself. 



Children announced that titanic acid was a constituent in this 

 mineral, which I could not "confirm; and Arppe' s explanation, 

 that some of the Arendal crystals contained minute particles of 

 titaniferous magnetite, no doubt accounts for this statement, 

 since, after extracting all magnetic particles from the powdered 

 mineral before analyzing it, Arppe found that no titanic acid 

 could then be detected in the purified silicate. 



XL V. Lecture Experiments to illustrate the Laws of Motion. 

 By Professor Robert Ball, A.M.% 



HAVE found the arrangements described in this paper 

 effective in demonstrating to an audience a few elementary 

 properties of gravitation and the laws of motion. It is certainly 

 true that a clear appreciation of the truth of these laws, so es- 

 sential for properly studying dynamics, requires some experi- 

 mental illustration to beginners. However satisfactory may be 

 the multitudes of indirect proofs of these laws with which the 

 more advanced student is familiar, it will hardly be denied that 

 it is a little difficult to demonstrate them directly. So far are 

 they from being axiomatic, that for centuries they were not be-? 

 lieved at all. 



* Berzelius, Jahrbuch, vol. xxii. (1842). 



t Phil. Mag. vol. xxvii. p. 123. 



X Communicated by the Author. 



