HISTORY OF THE ATOMIC THEORY, 17 



thermometers, making experiments on hygrometers of whip 

 cord, sending specimens of butterflies to Mr. Crosthwaite's 

 museum in Keswick, and afterwards engaging for a very 

 small sum to send him dried specimens of plants. Mr. 

 T. P. Heywood, of the Isle of Man, has in his possession 

 eleven volumes of his Hortus Siccus, The first is a thick 

 volume, containing the general title page, Hortus Siccus 

 sen Plantaram diversarum in Agris Kendal vicinis sponte 

 nascentium Specitnina, Opere et Studio Johannis Dalton 

 collecta, et Secundum classes et ordines disposita, 1790." 

 The other volumes are thin. They are not preserved with 

 the greatest care which collectors are capable of, but they are 

 a proof of great industry and of considerable attainment, even 

 in a branch of science which he did not profess. He supplied 

 Mr. Crosthwaite with a barometer and thermometer, although 

 not knowing how to make them. He nevertheless begins and 

 learns their faults by experience. A letter given by Dr. Henry 

 shews us how his knowledge stood at this time, and how also 

 he was in the habit of acquiring it. Speaking of the mercury 

 in the barometer, he says, " I intend to renew mine as soon 

 as convenient ; if thou do the same, be careful in undoing it ; 

 and attend to the cautions I give. Be sure to rub the inside 

 of the tube well with warm dry cotton or wool, and have 

 the mercury when poured in at least milk warm, for moisture 

 is above all things else to be avoided, as it depresses the 

 mercury far more than a particle of air does ; mine is, as I 

 have said, at least xVth of an inch too low, and yet it is ^clear 

 of air, and to all appearance dry ; but I doubt not but at- 

 tending to these precautions, which I knew nothing of when 

 it was filled, will raise it up to its proper height." 



At this time he felt uneasy, the sphere of his simple school 

 was too small, his impatience took the form of variety in his 

 pursuits, and he wandered over nearly every branch of 

 science. He seems now, although at a later time than 

 generally happens to young men, to have been "yearning 

 D 



