1894. ] NEW YORK ACADEMY OF SCIENCES. 237 
posed on the basis of common structural characters, but it now 
proves to be an unnatural one, although the term Bunotheria 
conveniently expressess the low state of evolution of these or- 
ders in the lower Eocene. The Mesoplacentalia are chiefly 
grouped upon common characteristics which appear to have 
been partly physiological. 
LITHIUM BICHROMATE AS A NEW REAGENT FOR 
HARDENING ADULT BRAINS IN THE 
GOLGI METHOD. 
By O. 8. Strona. 
The employment of the methods of impregnating the various 
elements of the nervous system introduced by Camillo Golgi 
have, in the hands of such investigators as Golgi, Ramon y 
Cajal, Kolliker, Retzius, von Lenhossék and many others, led to 
such rich acquisitions to our knowledge of the nervous sy stem, 
and thrown so much light upon its many obscure points that 
any improvement in its technique, even though slight, hardly 
requires an apology for its presentation. 
The principal defects of these methods may be said to be un- 
certainty of good results, inequality of the impregnation, and 
the expense, in the rapid method at least, involved in the use of 
so much osmic acid. The inequality of the impregnation is, as 
has often been pointed out,in many respects a valuable quality. 
It is on account of its remarkable property of impregnating 
only a limited number of the nervous elements in any one prep- 
aration that Golgi’s silver method has proved so invaluable in 
unravelling the complex structure of the various parts of the 
nervous system. In spite of this, however, it is at times desir- 
able to obtain more complete and uniform impregnations than 
are secured, except in rare instances. 
1. Sodium Sulphate Method. 
It occurred to me that this defect and the uncertainty of the 
method could be remedied to some extent by enhancing the pene- 
tration of the silver nitrate. This was attempted by mixing 
the silver nitrate solution with solutions of sodium or zine sul- 
phate. The following proportions were tried: Sodium sulphate 
8% 1 vol. + silver nitrate 1% 1 vol., sod. sulph. 6% 1 vol. + 
Silver nit. 4% 1 vol., sod. sulph. 1% 1 vol., + silver nit. 1% 1 
vol. Thespecimen was brought directly from the osmic-bichrom- 
ate mixture into one of these mixtures. Before washing in 
alcohol, the sulphate was removed by washing in pure silver ni- 
