Effect of Cold on the. Magnetic Fluid. 201 



in tlie vacuum, at Mr. Caiy's, I afterwards easily observed in 

 the needles in the open air, by lowering more and more the 

 temperature of the parallelopipeds, in consequence of many al- 

 ternate and thinner layers of the mixture. At all events, there- 

 fore, I would recommend the trial in vacuo, with layers like 

 the last, and with their ingredients reduced to the most conve- 

 nient thinness. 



But, waving the discussion till some practical obssrvations 

 give us more insight into the matter, let us rather relate how 

 the magnetic forces decreased under the exhaustion of the air- 

 pump, and liow they increased on the readmission of the air. 

 The decrease was slower than the increase ; but the exhaustion 

 too was slower than the readmission ; and it was the same with 

 respect to the progressive dimmution and elevation of the tem- 

 perature from 40 to nearly 30, and from 30 again to 40. But, 

 notwithstanding the greater rapidity of augmentation, the air 

 and the temperature once restored as before, the magnetic ac- 

 tion seemed not restored to the same degree. I do not know 

 whether this anomaly is to be deduced from an increase of 

 friction depending on the precipitation of aqueous vapours, or 

 from the protracted influence of the cold. How far could these 

 two circumstances affect the oscillatory laws of the magnetic 

 power when referred to the tw^o opposite agents in vacuo, viz. 

 the greater or less resistance of the medium combined with less 

 or greater degree of temperature, and tried at different inter- 

 vals through their different combinations ? and what is the 

 cause of such vaiying paralyses in needles ? It is not an easy 

 matter to answer the first of these queries, as depending very 

 much on the delicacy of instruments and experiments ; but as 

 to the second, it seems the cause of the magnetic paralysis is 

 not in the condensation of the air presenting an obstacle to 

 the magnetic currents, as the effect equally takes place in 

 vacuo: it seems the cause is not in the condensation of the 

 steel, as it is commonly known that the greater the density of the 

 steel, the more difficult, but the more great and tenacious, the 

 intensity of the same currents once established, and besides, 

 it could not so rapidly inci-ease and decrease ; therefore it is 

 only, or at least principally, the action of cold on the magnetic 

 fluid itself which produces the paralysis. Yes: as there are 

 ices of rougher fluids, so there are more refined ices of the 

 ethereal ones; and I)es Cartes's mecluuiical principles will ac- 

 count as well for those which we may see and touch and taste, 

 as for those which we can luit barely discover by their rela- 

 tions with the movement of the needles. 



Encouraged by the results of yesterday, I wished to try again 

 this morning, in the open air, the ettect of alow temperature 



Vol. 60. No. 293. Sept. 1822.. C c on 



