100 L. V. Pirsson — Datolite from Loughboro, Out. 



is something real and substantial, which has an independent 

 existence and, whatever it consists of, depends upon nothing 

 else. Whence we conclude, that any given vis viva, is of 

 determinate quantity of which none can disappear except it 

 reappear in the effect produced. Hence it follows at once, 

 that vis viva is always preserved, and so perfectly that what 

 inhered in one or many bodies before action is now, after 

 action, necessarily found in another or in several others except- 

 ing what remained in the first system. And this we call the 

 conservationem virium vivarum." 



Compare this with the modern statement : In any system 

 the variation of energy is equal to the external work done by 

 the system less the work done by external forces upon the 

 system. 



John Bernoulli was under no misapprehension as to the 

 importance of the principles he had stated. He says in sub- 

 stance : Whether bodies are regarded as communicating motion 

 to one another or whether one considers the various modifica- 

 tions of the motion of one and the same body depending on 

 its own force (where nothing can vanish without an equivalent 

 effect), " pro fundamento et principio universali poni debet 

 conservatio virium vivarum, hoc est iH'ms facultatis age?idi." 



That such men as Daniel Bernoulli and Euler should have 

 been deaf to teachings like these is impossible and they must 

 therefore have had an idea of the vis potentialis differing but 

 little from that which the words convey to modern ears. 



It would be interesting to know whether they really omitted 

 to show in any portion of their writings why they felt them- 

 selves at liberty to base investigations on the statement that 

 the potential energy of a strained solid is a minimum. Indeed 

 it appears to me that the few notes given above are sufficient 

 to indicate an opportunity for a physicist to write a most 

 interesting essay on the evolution of the potential up to the 

 date of Lagrange's memoir on the movement of a number of 

 mutually attracting bodies (1777) in which the potential seems 

 to have been used for the first time in an entirely general form. 



Washmo-ton, December, 1892. 



Art. XII. — Datolite from Loughboro, Ontario ; by 

 L.' V. Pirsson. 



The crystals of datolite which are the subject of this note 

 were sent to this laboratory for examination through the kind- 

 ness of Messrs. English <fc Co., of New York. They had been 

 forwarded to them by the owner and the occurrence is the 



