FOR THE SCIENCE OF CALORIFIC RADIATIONS. 529 
tion of a new system of nomenclature. I will explain the prin- 
ciples which have guided me in its production. 
The comparison between the properties of heat in its ordinary 
state and in the radiant form, furnishes several distinctive cha- 
racters between these two great classes of natural phenomena. 
In fact, ordinary heat is propagated slowly; it traverses any 
direction whatever, straight or sinuous; it undergoes greater or 
less alteration in the velocity and direction of its motion as well 
as in its own energy, when we agitate the medium through which 
the propagation is effected. Radiant heat, on the contrary, leaps 
the whole extent of the medium in an imperceptible instant ; it 
goes only in a straight line, and always preserves the same in- 
tensity and the same direction whatever may be the state of. 
_ repose or of motion of the particles of the medium traversed. 
Each of these three properties belonging to the two methods of 
_ transmission, namely, the velocity of the calorific flow, its direc- 
tion, and its connexion with the agitation of the medium, takes, 
in one of the two cases, a contrary character to that which it 
affects in the other. All may therefore serve as a basis to the 
system of nomenclature which we wish to establish; but the 
corresponding Greek or Latin words, taken as radicals, scarcely 
lend themselves to the formation of a language capable of ex- 
pressing with ease, and with a suitable precision, the different 
actions of bodies on calorific radiations. It is nearly the same 
with regard to the expression ray of heat, upon which we 
might equally found the new thermological nomenclature, if we 
Were not every instant stopt by the hardness and the com- 
plication of its derivations*. Happily, besides the characteristic 
_ * If we were content with the substantive éx7ziv (ray), the greater part of 
these difficulties might be avoided, and perhaps a very simple nomenclature 
might be formed; but we should fall into the very serious inconvenience of 
making the denominations applicable to all kinds of rays, which would not fail 
to occasion frequent mistakes, and thus introduce a real confusion into the 
_seience. And here the demonstration immediately follows the principle ; for the 
confusion is about to begin with some new names which have lately been pro- 
hase in meteorology. In fact, the members of the Committee of the Royal 
ociety of London who were charged with the scientific instructions for Captain 
_ Ross's voyage, call an instrument which serves to measure the heating power 
of the solar rays an actinometer. M. Pouillet describes under the same name 
his thermoscopic apparatus, designed for the exploration of the nocturnal cool- 
ing of bodies under a serene sky. Sir J. Herschel indeed uses actinograph, a 
‘denomination quite analogous, to indicate a very ingenious little machine of 
his invention, by means of which, according to him, the different degrees of 
light which succeed each other during the day are measured. Now why should 
not some other physicist wish to apply, and with as good reason as his prede- 
cessors, the word actinology, not to such and such a branch of radiant heat, 
Penne 
