276 
NATURE 
| Feb. 2, 1871 

HEAT SPECTRA 
NOTICE in NATUuRE, vol, i., p. 28, an account of 
some very important researches on Heat Spectra, 
made by the late Professor Magnus; and I am gratified 
to think that some observations which I made on the sub- 
ject in the years 1858 and 1859 were confirmed by this 
eminent German philosopher. 
In a paper read before the Royal Society of Edinburgh 
in 1858, I showed that rock-salt absorbs heat radiated 
from rock-salt in larger quantity and more powerfully 
than other kinds of heat; and also that the amount of 
absorption of rock-salt for heat increases with the thick- 
ness of the absorbing plate. These are the fourth and 
fifth results of Professor Magnus. His next result is very 
interesting, namely, that the high diathermancy of rock- 
salt does not depend on its small absorptive power for the 
different kinds of heat, but on the fact that it only 
radiates, and consequently only absorbs, heat of one kind ; 
while almost all other bodies at the temperature of 150° 
F., emit heat which contains only a small fraction or none 
of those rays which are given out by rock-salt. 
Certain experiments which I made in 1859 lead me to 
think that Professor Magnus was quite justified in his con- 
clusion that the heat radiated and absorbed by rock-salt 
is a peculiar kind of heat. These experiments, which are 
described in the Transactions of the Royal Society of 
Edinburgh, are as follows. I tested the quality of the 
heat radiated by rock-salt at 212° F. by transmitting it 
through three different screens :— 
a A screen of mica. 
B One of mica split by heat. 
y One of glass. 
It was found that a mica screen, which passed about 31 
per cent. of ordinary lampblack heat, passed only 18 per 
cent. of rock-salt heat ; or if we call the proportion of 
black heat passed by the mica 100, that of rock-salt heat 
will be 58. Again, it was found that while 20 per cent. of 
lampblack heat passed through a screen of split mica, ; 
the proportion of rock-salt heat transmitted through the 
same screen was only 15} per cent. These numbers are 
to one another as 100 to 76. 
Lastly, with respect to a glass screen, calling the pro- 
portion of lampblack heat which passed 100, that of the 
rock-salt heat which passed the same screen was 57. 
On these results I remark as follows :—It is already 
well known that rays of great refrangibility, or small wave 
length, pass through glass or mica more readily than those 
of an opposite character. The difficulty with which rock- 
salt heat penetrates these substances as compared with 
ordinary heat, might therefore lead us to infer that the 
wave length of this heat is greater than that of ordinary 
lampblack heat. 
If we now look to the relative transmission of the two 
descriptions of heat through mica split by heat, we see 
that the facility of transmission is yet in favour of ordinary 
heat, but not so strikingly as with a screen of common 
mica. This will be seen from the following table :— 
Transmission of 
Rock-salt Heat 
Transmission of 
Nature of Screen Ordinary Heat 
at 212° at 212° 
AYER Be ery Poses 100 58 
Mica split by heat . 100 76 
Compare this with the following table, deduced from 
the results given by Professor Forbes :— 
Transmission of Heat Transmission of 
Nature of Screen from Blackened Brass Biack Heat 
at 700° F. at 212° F. 
Mica ‘ors inch thick 100 52 
Mica split by heat . 100 64. 
. From a comparison of these two tables, it will be seen | 
that, as tested by the two substances, mica and mica split 
by heat, rock-salt heat at 212° F. bears to ordinary heat of 
that temperature a relation similar to that which ordinary 
/ heat at 212° bears to heat at 700°; that is to say, that just 
|} as heat of 212° has a greater wave-length than heat of 
7oo°, so rock-salt heat at 212° has a greater wave-length 
than ordinary heat at that temperature. And the surface 
stoppage produced by splitting the mica, telling most 
powerfully upon heat of high temperature, or small wave- 
length, while the stoppage by substance is in the opposite 
direction, we see how the one effect tends, to a certain 
extent, to neutralise the other, rendering the proportions 
of different kinds of heat passed by split mica more nearly. 
alike than those passed by ordinary mica. In connection 
with these remarks I may state that neither the radiation 
nor the absorption of a plate of rock-salt is sensibly in- 
fluenced by roughening its surface with emery paper. 
All these experiments concur in showing that heat from 
rock-salt possesses very great wave-length, and probably 
heat from a thin plate of this substance, at a low tempera- 
ture, may be found to possess a greater wave-length than 
any other description of heat which can be exhibited. 
The observations of Professor Magnus with respect to 
the nature of the heat from potassium chloride are very 
interesting ; unfortunately I did not make any experiments 
on this substance, but I did on some others in the shape 
of powder. 
I found that the comparative radiation at 212° was as 
follows :— 

From lamp black - 100 
Alum in powder > i OS 
White‘sugar. 5). <, «=a, 0 ep Oo 
Sulphate of potash : 88-1 
Nitrate of potash 86°7 
Table'salég 2 72, Peso 83:1 
The experiments of Prof. Magnus on the reflection of 
heat are also of the very greatest importance, and they 
strengthen the evidence (already overwhelming) in favour 
of that view which regards light and heat as varieties of 
the same agent differing in nothing except wave-length. 
BALFOUR STEWART 

ON THE METHOD OF. ASSAYING SILVER 
AS CUNDUCTED IN THE INDIAN MINT* 
BY DR. H. E. BUSTEED, OFFG. ASSAY MASTER 
“] HE method of assaying silver, as now in use in H.M.’s 
Indian Mints, is one peculiar to them ; it was introduced 
into the Calcutta Mint about the year 1850, and thence extended 
in course of time to those of Bombay and Madras. 
Ic has been favourably reported on and described more or 
less in detail as an official duty by various assay officers to local 
Mint authorities in India ; but beyond this it would appear that 
no attempt has been made towards giving publicity to the prac- 
tical working of the process, or to making generally known the 
laboratory details of this method of assay. 
I* has been suggested to the writer that some such attempt 
now would be not only interesting but useful, as after twenty 
years’ experience of it, the assay offices inthe Indian Mint must 
be in a position to assign its true value to a method which has 
been used for the assay of an immense importation and coinage 
of silver bullion. To render it more generally intelligible, and to 
show wherein the process about to be explained contrasted with 
those in more general use, Dr. Busteed very briefly adverted to 
the principles on which those processes depend for their results, 
omitting details and technicalities. In modern acceptance, the 
principal duty of an assayer is to ascertain the proportion of the 
precious metals present in any sample of mixed metal submitted 
to him for examination, so that from the result of his investiga- 
tion the proper value may be assigned by calculation to the mass 
which the sample is supposed to represent. 
This the assayer effects by separation of the precious metals 
from the coarser ones. The most ancient means of effecting 
this was by the method of cufellation. He explained the 
principle of this method, what skill and experience it required on 
the part of the operator, and how it still fell short of accuracy in 
its results. 
* From the Proceedings of the Asiatic Society of Bengal. 

