ON ISOMERIC CRESOLS AND THEIR DERIVATIVES, 73 
with a view to affording facilities to persons desirous of becoming teachers 
of Physics, of familiarizing themselves with the most efficient methods and 
of gaining experience in them, the Council of the British Association should 
invite the leading teachers of Physics in the universities, colleges, and schools 
of the United Kingdom to allow such persons, under suitable regulations, to 
be present at the instruction given by them, and, when practicable, to act as 
temporary assistants. The Committee do not hereby mean that aspirants to 
the teaching function should be encouraged to drop in at random to hear a 
lecture by any established teacher who may happen to be within reach ; the 
kind of attendance they have in view would be systematic, and continued for 
not less than some moderate period of time, such perhaps as two or three 
months, agreed upon at starting. They believe that the benefits which 
might result from the adoption of such a plan are very great; the advantages 
to those who might avail themselves of it are obvious; and while teachers of 
established success would have a chance of spreading widely their methods 
of instruction, and, in fact, of founding schools of disciples, the stimulus to 
exertion, afforded by the consciousness that they were being watched by men 
who were preparing themselves to occupy positions similar to their own, 
would be of the most efficient kind. 
Preliminary Report of the Committee, consisting of Dr. ARMSTRONG 
and Professor THorre, appointed for the purpose of investigating 
Isomeric Cresols and their Derivatives. Drawn up by Dr, Henry E. 
ARMSTRONG. 
A number of isolated observations have shown that the so-called cresylic 
acid from coal-tar contains both para- and ortho-cresol, but a satifactory 
examination of the crude product which would enable us to say that it con- 
sists of these two modifications alone has not hitherto been made ; moreover, 
supposing it to contain only these two isomerides, no method is at present 
known by which it is possible to separate them and obtain each in a state of 
purity. In conjunction with Mr. C. L. Field your reporter has therefore 
sought, in the first place, to ascertain what are the constituents of ordinary 
eresylic acid; and, in the second, to devise a method of separating the isomeric 
cresols. 
The method of examination employed is as follows :—The cresylic acid is 
heated with an equal weight of concentrated sulphuric acid for 15-20 hours 
at about 100°; the resulting mixture of sulpho-acids is then thrown into 
water and neutralized with baric carbonate, and to the solution separated 
from the precipitated baric sulphate baric hydrate solution is added as long 
as a precipitate is produced. The basic baric salt of paracresolsulphonic acid 
thus precipitated is separated from the liquid, decomposed by a slight excess 
of sulphuric acid, the excess of sulphuric acid is removed by plumbic car- 
bonate and hydric sulphide, and the solution of paracresolsulphonic acid thus 
obtained neutralized with potassic carbonate. On concentrating the result- 
ing solution potassic paracresolsulphonate, C, H, SO, K, 20H,, separates out 
almost in a state of purity. 
The solution filtered from the basic baric salt is treated with sulphuric 
acid, and thus at least two thirds of the barium present removed as sulphate ; 
