ON THE CONDUCTIVE POWEES OF VAEIOUS SUBSTANCES. 
841 
(B, B') placed in them should be as nearly as possible under the same conditions 
during the experiment. The small orifice between them admits the thermometer T, 
for the determination of the temperature f,, while is determined for each block sepa- 
rately. AVhen this has been done, the two blocks are interchanged, each being thus 
placed in the hole previously occupied by the other, and the experiment repeated. The 
mean results of the two experiments have been adopted. The discrepancies, though 
small, are sufficient to show the necessity of the precaution where considerable accuracy 
is required. 
29. It would be useless, I conceive, to record the details of all the numerous experi- 
ments made for the purpose of simply determining the conductive powers of particular 
substances. They were made in the manner already described (art. 26), and when the 
apparatus was once arranged, required little but patience to wait for the final indica- 
tions of the thermometers. Such at least was the case when the steam from boiling 
water was used to produce the temperature of the lower surface of the block ; when 
any other temperature was employed, constant attention was necessary to preserve it as 
steady as possible. 
It may sufiice to give the details of one experiment of this kind made with high tem- 
peratures, from which it may be understood how far the steadiness of the temperature 
could be maintained. In almost all cases, however, the same temperature, tliat of 
boiling water, was used for in the experiments now spoken of, in order that they 
might all be made under nearly the same conditions with Tespect to temperature. The 
experiments made to compare the conductive powers of two blocks, for the purpose of 
ascertaining the influence of pressure, moisture, discontinuity, &c., require greater care ; 
and I have therefore thought it right to give all the principal ones of this kind in suffi- 
cient detail to enable the reader to judge of the evidence which they afford. The sym- 
bols used have the same signification as heretofore (art. I, &c.). I repeat them here 
for the convenience of more immediate reference. 
temperature of the mercury in contact with tlie lower surface of the cylindrical 
block B, on which the experiment is made ; 
that of the mercury on its upper surface ; 
f\— that of the tipper surface of the surrounding or outer block A ; 
T = that of the air immediately above the blocks A and B ; 
/< = the length of the cylindrical block B ; 
k = conductive power of B ; 
c — radiating power of mercury. 
It will be observed that in some of the following experiments the recorded value of f.,, 
is greater than that of in opposition to the remark in the last paragraph of article 20. 
In such cases the temperature really corresponded to points at somewhat a lower level 
in the outer block A than the upper surface of B to which corresponds. I only 
observed to assure myself of there being no amount of lateral transference of heat 
