Differential Thermometer. 237 



to this plate, I observed a very clumsy and distorted repre- 

 sentation oi' my differential thermometer, placed by the side 

 pf another figure, bearing a general reseniblance to it, and 

 purporting to be a copy of Van Helmont's sketch. Anti- 

 cipations, such as are here insinuated, have no doubt oc- 

 curred in the history of scientific disctiverv; but there has 

 been so strong a propensity of late to bring forward allega- 

 tions of ih.is nature on the slightest grounds, that I was 

 tempted, for the first time, to examine the original work. 

 I soon perceived, however, that Van Helmont's description 

 and fiiiure were essentially different from the representation 

 which Sir Humphry has taken the trouble to give. In 

 fact, the" curious instrument' described by the alchetnist is 

 no other than the common air-thermometer of Sanctorio or 

 Drebbel, invented more than forty years before his death; 

 only, for the sake of easier carriage, shaped like a syphon, 

 the lowtr end being bent upwards, and terminating in a 

 spherical cup, with a smuU orifice — one of the forms which 

 it had from its earliest introduction. A learned person of 

 the name of Heer imagining, it seems, as Sir Humphry 

 has since done, that the instrument was absolutely closed, 

 had proceeded to admire the perpetual motion of the con- 

 tained liquid, and next to wonder at its gradual disappear- 

 ance, which be sagely regarded as an irrefragable proof of 

 the conversion of water into air. To this Van Helmont 

 replied, that the action of the machine no more produced 

 pcr|)etual motion than the changing of a weather-cock ; and 

 urged that, if both the balls were shut, the liquor must, 

 under all the changes of external heat, remain stationary, 

 being pressed bv tlie equal and opposite elasticities of the 

 secluded masses of air — a siaiemeiu which incidentally in- 

 volved the principle of the difiLTenlial thermometer, but 

 which the author never once dreamed of reducing to use. 

 On the contrary, he calls his rival, in the uncourtly lan- 

 guage of those times, " an idiot," and charges him witK 

 " stupidity," for not perceiving that the instrutnent had aii 

 operture, only slightly shut with a stojjper, and not her- 

 metically sealed, as it is most incorrectly figured in the 

 " Elements of Chemical Philosophy." 



I will not suppose that Sir Humphry intentionally mis- 

 represented the meaning of Van Helmont ; but then it fol- 

 lows, that either he had not read the passage to which he 

 refers, or must have satisfied himself with a very superficial 

 and careless infipection. This precipitancy is the more to 

 be laiiienied, a» it may possibly beget suspicion of want 



of 



