VOL. XL.] PHILOSOPHICAL TRANSACTIONS. 223 



even Sir Isaac Newton was of opinion, that it was necessary the earth should 

 be more dense towards the centre, in order to be so much the flatter at the 

 poles; and that it followed from this greater flatness, that gravity increased so 

 much the more from the equator towards the pole. 



New Experiments on Ice. By the Abb^ Nollet* F. R. S. at Paris. N° 449, p. 307 . 



1. Ice that begins to melt, and water that begins to freeze, have always the 

 same degree of cold. 



2. Thiit cold may be increased by a mixture of salts. 



3. It has been thought for a long time, that saltpetre was most fit to increase 

 the cold of ice; but experiments have shown, that few salts increase cold so 

 little as that salt. Mix one part of fine saltpetre with two parts of beaten ice, 

 and Mons. Reaumur's thermometer will descend in it but 3-f degrees below the 

 freezing point. 



What had caused this mistake, is, that people generally made use of salt- 

 petre of the first or second melting, as being the cheapest; but that salt- 

 petre, not being purified, contains a great deal of sea- salt ; and it was in pro- 

 portion to the quantity of the sea-salt that the effect was the greater. 



From this last observation, one may deduce an advantageous method for 

 trying gunpowder; for as of the three ingredients of which it is made up, 

 salt-petre is the only one that can increase the cold of ice; if one pari of gun- 

 powder, or a little more, be mixed with two parts of ice; and it increases its 

 cold more than 3i degrees, it is a sign that the salt-petre contained in it is not 

 well purified; and the best powder will be that which least increases the cold of ice. 



4. Sea-salt, that is the Bay-salt, which is commonly used at table in France, 

 and that which is immediately taken from the mines, called sal gemmae, usually 

 gives the greatest degree of cold; for pot-ash gives sometimes a little more, 

 but generally less. Sea-salt mixed with ice in the abovesaid proportion, gives 

 15 degrees of cold on Mons. Reaumur's thermometer, and sal-gem. 17. 



5. Ashes of green wood 3 deg. 



6 Sea-coal 



7 Vitriol 2 



8 Tartar 10 



9 Common pot-ash (in French called sonde ordinaire) 3 



10 Pot-ash made of se^ weed 11 



This last pot-ash may be substituted instead of sea-salt, for making ice- 



* The Abbd Nollet acquired a great degree of celebrity by his lectures and writings on experi- 

 mental philosophy, and particularly by his experiments and observations on electricity. He was a 

 member of the French Academy of Sciences, of the Royal Society of London, and of various other 

 ficientiflc institutions. He died at Paris in 1770, at the advanced age of 70. 



