168 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXX.) 
magneciystallic, set, in the magnetic field, with the optic axis axially, as Tyndall and 
Knoblauch describe*. As wholes they are attracted in every position, whether sur- 
rounded by air or carbonic acid (more strongly in the latter than in the former) ; and 
when free to revolve they set with the optic axis axially, even in solutions of iron. 
But there seems no reason why calcareous spars, intermediate between these and the 
sphere, should not be found by search; nor any reason to doubt that, being crystal- 
lized, they would be magnecrystallic. The further suggestions of hypothesis are, 
however, not very clear, inasmuch as we are not quite sure, without other experiments, 
whether such bodies may be accepted magnetically as simple bodies, or whether the 
optic axis would always point axially or equatorially as the body was paramagnetic 
or diamagnetic in relation to space ; or whether the body would disappear from 
the list of magnecrystals altogether. For suppose such a body coincident in its 
general magnetic condition (as when pulverized or amorphous) with carbonic 
acid or space; and that being in its crystallized condition it should be magne- 
crystallic, and when formed into a sphere should, according to the results just 
given (3381.), point in the same direction and with the same degree of force, 
whether surrounded by water, or carbonic acid, or solution of iron ; — what direction 
should the optic axis of such a sphere take? It cannot take that of the pure cal- 
careous spar, and also that of the small rhomboids of ferrocarbonate of lime, for 
they are at right angles to each other in the magnetic field ; neither does any reason 
appear why it should take one more than the other. I would willingly think that 
some valuable considerations and evidence regarding the true zero of magnetic 
force would arise in the investigation of this matter; but the former results with 
different media make me fear that the subject, when closely examined experiment- 
ally, will resolve itself into something of less importance. This matter is carried a 
little further in the relations of temperature (3416.) ; for if we consider a true zero 
as independent of temperature, then one temperature may be assigned for it as well 
as another; and it will be seen that, in the mixed substance presented by ferrocar- 
bonate of lime, we have a body that can be placed as non-magnecrystallic in carbonic 
acid at a given temperature ; whereas at higher temperatures it is magnecrystallic 
as carbonate of lime, and at lower temperatures as carbonate of iron. 
§ 39. Action of heat on magnecrystals. 
3394. Heat affects the degree and, perhaps, the disposition of the induced forces 
in magnecrystals (2569-2573.). There is as yet little or no experimental evidence 
bearing upon this subject, so that the following contribution for temperatures between 
0° and 300° Fahb. may be acceptable. Some new arrangements of apparatus were 
required, the following brief description of which will probably be sufficient. 
3395. Baths for the application of heat and cold were necessary. One, frequently 
* Philosophical Magazine, 1850, vol. xxxvi. p. 178. 
