168 Mr. M. Ponton on certain Laws 
and this having been accomplished, the mutual relations of these 
extrusions, whether in the same or in different media, and the 
laws by which they are governed, may be advantageously studied. 
This is indeed the most important branch of the inquiry; for 
unless it can be shown that the extrusions follow some determi- 
nate law, the general formula becomes indefinite. 
A careful analysis and comparison of the extrusions of the 
fixed lines in various media have brought to light the following 
general principles and laws. 
The extrusive power of a medium may in every case be defined 
to be “a property in virtue of which the medium transfers a 
small portion of motive energy from one part of the spectrum to 
another.” 
The media hitherto examined may be distinguished into two 
great classes—Regular and Peculiar, the former being consider- 
ably the larger of the two. As all media, however, are greatly 
affected by temperature, it may sometimes happen that a medium 
may be regular at one temperature and peculiar at another. 
These two classes shall be separately considered and described. 
The following is a list of all the regudar media hitherto ex- 
amined :— 
Solution of Nitrate of {Caleareous spar, both | Oil of anise, temp. 20° 
bismuth. rays. and 13°. 
Water. Oil of sassafras. Creosote. 
Sol. Subacetate of lead. | Sol. Nitrate of lead. Crown-glass. 
Sol. Nitrate of mercury.|Sol. Muriate of am- | Rock-salt. 
Sol. Sulphate of soda. monia. Arragonite, three axes. 
Sol. Muriate of baryta. | Nitric acid. Quartz, both rays. 
Sol. Superacetate oflead. | Sol. Muriate of lime. | Sulphuret of carbon. 
Sol. Nitrate of potash. | Sol. Potash. Flint-glass. 
Sol. Sulphate of mag- | Oil of turpentine. Topaz, three axes. 
nesia. 
In all regular media the transfer of motive energy takes place 
from the terminal to the central parts of the spectrum ; and they 
are therefore medio-positive. The undulations corresponding to 
the lines D, E, and F are always quickened by the extrusive force ; 
consequently their wave-lengths within the medium are length- 
ened. Hence for these three the formula is always 2 —a+ev=u. 
On the other hand, the undulations corresponding to the fixed 
lines B, C, G, and H are always retarded, or their wave-lengths 
within the medium are shortened. Hence for these four the 
pels y ‘ 
formula is mars In every case the motive energy gained 
by the one set of waves is exactly balanced by the loss sustained 
by the others ; so that the sums of the positive and negative ex- 
