124 
course be prevented; for this purpose the preparation may be sur- 
rounded wich a frame of soluble glass or paraffin wax as is usually 
done in histological technics. 
5. Some general observations. 
In chemical work other metals and acids than those we added 
will be met with in the fluid containing the sulphate. It will have 
to be determined by an intentional addition of these substances in 
what degree they influence the volume of the precipitate. These 
researches fall outside the scope of this paper. But it remains to be 
settled to what extent the noxious influence of proteid can be 
neutralized. Experience has taught us that this can be easily done 
by ultratiltration. A filter, drenched with a 5°/, sol. of celloidin in 
alcohol and ether and subsequently treated with water easily keeps 
back the proteid. 
As regards the degree of accuracy of our method we may give 
the following calculation: 2 ec. of Na,SO, 10 aq. 4°/, contain 
0.02386 grammes of SO,..The volume of BaSO, gives by these 
averages 81.5 divisions. Lach division of our tube corresponds, there- 
. 0.02386 hea oe Pe 
fore, to TR 0.000294 grammes of SO,. Since in accurate work 
‚9 
no greater mistake is made than one division, the method is accurate 
to 0.000294 grammes of SO, 
The object of this paper is twofold. First it intends to supply a 
simple method enabling us to make in an easy manner highly accu- 
rate determinations of very slight quantities of SO,. Secondly it 
aims at giving an impulse to the further development of a new 
method of quantitative analysis, which certainly deserves to rank 
with both the micro- and macro-gravimetric method, beeause it is 
simple and accurate, saves time, allows us to work with slight 
quantities, and is moreover particularly adapted to series of deter- 
minations. These qualities lately became manifest at the determination 
of potassium, and some years ago in the study of an equilibrium 
reaction’). The great obstacles to be encountered will especially 
consist in the difficulty to discover the conditions under which a 
precipitate is obtained giving at all times the same microscopic view. 
At the present moment tlfis can be effected only empirically ; indeed, 
as far as I am aware, next to nothing is known of the forces which 
underlie the varieties of crystal shape and crystal size of the same 
1) HAMBURGER and ARRHENIUS. On the natue of precipitin-reaction. These 
Proceedings, meeting of May 26th 1906. 
