412 PHILOSOPHICAL TRANSACTIONS. [aNNO 1/83. 



struments used were simply thermometers, except the apparatus f and g, fur- 

 nished by Mr. Cavendish. 



The first 5 experiments were made according to the directions sent by the So- 

 ciety, to obtain the point of congelation ; as are the 2 next, to endeavour to 

 ascertain the greatest degree of contraction mercury is capable of; then follow 2 

 experiments made in a different manner by Mr. H.'s own suggestion ; and lastly, 

 an account of mercury frozen in the open air without the aid of any artificial 

 cold, which will be found to corroborate the preceding experiments, and deter- 

 mine the exact point of congelation to be at 40° below the cypher. Mr. H. here 

 inserts a letter from the ingenious Dr. Black, of Edinburgh, who favoured him 

 with some remarks on the experiments he made in 1775 to freeze quicksilver, 

 and first suggested this method of ascertaining the point of congelation. 



Dr. Black's Letter referred to above. Dated Edinburgh, 5th Oct. 1 779. 

 I have read with pleasure the experiments made at Hudson's Bay, on the con- 

 gelation of mercury, and observe that the author has succeeded perfectly in effect- 

 ing it ; but could not determine with precision what degree of cold was necessary 

 to produce it. This, however, does not surprize me, as I have always thought 

 it evident, from Professor Braun's experiments, that this degree of cold cannot 

 be discovered conveniently by congealing the mercury of the thermometer itself. 

 I shall not here give my reasons for this opinion ; they would lengthen out this 

 letter too much. I shall only propose what appears to me the proper manner of 

 making the experiment, which is as follows : provide a few wide and short tubes 

 of thin glass, sealed at one end and open at the other ; the wideness of these 

 tubes may be from half to three-quarters of an inch, and the length of them 

 about three inches. Put an inch or an inch and half depth of mercury into one 

 of these tubes, and plunging the bulb of the thermometer into the mercury, 

 set the tube with the mercury and the thermometer in it into a freezing mixture, 

 which should be made for this purpose in a common tumbler or water-glass ; 

 and, n. b. in making a freezing mixture with snow and spirit of nitre, the quan- 

 tity of the acid should never be so great as to dissolve the whole of the snow, 

 but only enough to reduce it to the consistence of panada. When the mercury 

 in the wide tube is thus set in the freezing mixture, it (the mercury) must be 

 stirred gently and frequently with the bulb of the thermometer ; and if the cold 

 be sufficiently strong, it will begin to congeal by becoming thick and broasy like 

 an amalgam. As soon as this is observed, the thermometer should be examined 

 without lifting it out of the congealing mercury ; and I have no doubt, that in 

 every experiment, thus made, with the same mercury, the instrument will 

 always point to the same degree, provided it has been made and graduated with 

 accuracy. 



