( fi49 ) 
mal form and variants may be kept constant, as it seems for an 
unlimited length of time. 
3. All the variants vary in a way analogous to that of the normal 
form, thus, the awrafus-variant produces an <rmra&&?-slimevariant, 
which must be considered as a gain-variant, and an a/fow-variant, which 
must be taken for a loss-variant. 
The natural variety B. Kieliensis, which approaches the auratas- 
variant, also varies in an analogous way. The variation thus seems 
to be directed or orthogenetic. 
4. Gain-atavism in loss-variants and loss-atavism in gain-variants, 
can be obtained with certainty by determined experiments. Qualita¬ 
tive variants, too, may give rise to atavism. 
5. The experimental variants of B. prodigiosus have not yet been 
found in nature. From another bacterium, Bacillus herbicola , a variant, 
took rise which I had before repeatedly isolated from nature and 
which I had taken for quite another species. 
6. The variants of prodigiosus, and this holds good for many 
other microbes also, differ from each other and from their stock 
forms in the same way as closely related natural species or varieties do 
among each other. But their disposition to atavism is much more 
pronounced. 
7. The sub-variants, e. g. the rose variants of different colour- 
intensity, arise in the same way as the chief variants and possess 
the same degree of constancy. 
Physics. — “ Researches on magnetization at very loro temperatures.” 
By Pierre Weiss and H. Kamerlingh Onnes. Communication 
N°. 114 from the Physical Laboratory at Leiden. 
§ 1. Object of the research ; results. 
a. Introduction. The extension of Lange vin’s : ) kinetic theory of 
magnetism to all ferromagnetic phenomena by means of the hypothesis 
of the molecular field *) rendered the testing of deductions from this 
hypothesis by experimental data of great importance. The first results 
of this comparison were very encouraging; in some respects a 
remarkable correspondence was found. For instance the curves 
1 ) L angevin. Ann. Chim. et Phys. 8 Ser. t. 5, p. 70; 1905. 
2 ) P. Weiss. Journ. de Physique 4e S6r. t. VI, p. 661; 1907. 
