' On the Solulility of alkaline and earthy Salts, 419 



that, at low temperatures, a difference of several degrees is ne- 

 cessary to produce motion in the fluid. The evaporation of the 

 warm water, and of the surface of that in the jar, would tend to 

 cool the top of the fluid at a quicker rate than it could be sup- 

 plied from the middle, till the motion had become accelerated ; 

 when the supply from the middle would exceed the expenditure, 

 and cousequeiitly the temperature at the top would increase. 



The sixth experiment only proves that, the thermometers being 

 20 inches distant from each oilier, and the upper part of the 

 water at 33^, the lower part might be heated to 39^ without 

 causing- motion in the fluid ; for that there was no motion is 

 evident, because when the upper thermometer waa at 3G"5° the 

 water was freezing at the middle of the height of the jar. 



Dr. Hope seems to have been aware that part of the facts 

 might be accounted for by the effects of the cohesion and inertia 

 of the water. The fourth experiment led him to believe that these 

 forces had little effect. But that an imperceptible change of 

 density should overcome the effects of cohesion and inertia, ap- 

 pears as much an anomaly as the expansion of water by cooling; 

 for the direct cohesion of a square inch of water is not less than 

 40 grains troy: add to this the effect of inertia, and it will not 

 appear surprising that the small increase of bulk which water 

 under 40° ac(iuires by increasing its temperature Q\ should be 

 necessary to put in motion. 



Had Dr. Hope used three thermometers in the two last ex- 

 periments, it would have thrown some additional light on the 

 subject : at present we have many reasons for doubting the fact, 

 that water is condensed by the addition of heat. 



I am, sir, vour most obedient servant, 



T. 



LXXVII. On the Solulility of alkaline mid earthy Salts, as 

 affected by an Excess of the Acid. By A Correspondent. 



To Mr. Tilloch. 



Sjr, — In prosecuting a set of experiments on the alkaline and 

 earthy salts, the anomaly which takes place on their solution, 

 some in an increased and others in a decreased ratio, by an ex- 

 cess of acid, as are observable in the tartrate and super tartrate 

 of potass and the sulphate and super sulphate of potass, could 

 not escape my observation. Potass with one dose of tartaric 

 acid forms a salt easily soluble ; an extra dose being added, in- 

 fttead of precipitating in the neutral state, it combines with it 

 and falls down : but a saturated solution of this less soluble and 

 super salt, when treated with the crystalline acid, affords a pre- 

 D d 2 ^ cipitatc 



