Physics. 263 
We shall estimate the quantity of heat thus set free by the number of 
kilogroms of water which it would heat 1° C. The quantity of heat 
necessary to raise one kilogram of water one degree is called a unit of 
heat. 
It has been established by numerous experiments that the combustion 
of one kilogram of dry charcoal in oxygen, so as to form carbonic acid, 
yields 7200 units of on which fact may be briefly expressed by saying 
that charcoal furnishes 7200° of heat. 
Superior coal yields 6000°, perfectly dry wood from 3300° to 3900°, 
sulphur 2700°, and hydrogen 34 ee. of hea 
ait 
Ace 
on the quantity of matter which is pect ia and not on the 
under which the burning takes place. The same pe a of jae is 
given out whether the combustion proceeds slowly or quickly, in atmo- 
spheric air or in pure oxygen gas. If in one case a metal be burnt in 
air and the amount of heat directly measured, and in another instance 
€ same quantity of metal be oxydized in a se higaa apne the heat 
being developed in some o er place—say, the wire which conducts the 
waka »—in both of these sila 2 the same Beall ne of Teas will be 
observ 
The same law also holds good for the production of heat by mechan- 
ical means. The amount of heat obtained is only dependent on the 
which i of power pal bigs and is unite independent of the manner in 
i ower h 
me be found ay ‘taehleiok It is of no consequence whether this ae 
consists in the compression, percussion, or friction of bodies. 
The amount of mechanical work done by a force may be ex xpressed 
by a weight, and ms height to which this weight would be raised by 
€ same force. mathematical expression for “ work done,” that is 
to say, a measure for this work, is obtained by mulipying the neinss 
expressed in feet or other units by the number of pounds o tO 
lifted to this height. 
performed. This measure we shall calla Parsi and Das for 
it the symbol Km 
‘ echanical work may Hbearen | be Besiipy gine by ie pe mancsy bisa 
Y a given weight in in state >? 
The hil Nana's Redigerea bis ye by the product sheeal by the multi- 
css of the weight by the square of its velocity. The first. method, 
Owever, because it is the more convenient, is the one usually adopted ; 
and the numbers obtained therefrom may easily be expressed in other 
units. 
The heat ee raise 1 kilogram of water 1° C. will heat 1 Ib. av. of 
wasn ano 
kilogram == 2°2045 Ibs. av., it 
Silene that sper ag esi ee pom) 
