SCIENTIFIC RESEARCH IN PRACTICE. 255 



itself gives a faultless product, will, when mixed, give rise to 

 disease in the leer. He made these experiments with the two 

 species of Carlsberg bottom-yeast No. 1. and No. 2 ; in one set 

 of experiments the pitching-yeast consisted chiefly of No. 1 

 with a small admixture of No. 2, and in the other set, the 

 reverse was the case. It was found that in all cases the small 

 quantity of the admixed yeast, whether No. 1 or No. 2, made 

 the beer less stable as regards yeast-turbidity, than when the 

 chief constituent of the pitching-yeast was employed alone. 

 Thus the two cultivated yeasts under these conditions behaved 

 in such a manner as to produce effects similar to those brought 

 about by the wild yeasts (Sacch. Pastorianus III. and Sacch. 

 dlipsoideus II.). We can therefore only obtain true uni- 

 formity in working when a suitable species has been obtained 

 from the yeast-mass by systematic selection, and has been culti- 

 vated by itself. 



The systematic studies which HANSEN has carried on for 

 many years on the constancy of the characters of different species 

 of yeast, have proved that, under the conditions of the brewery, 

 they commonly only undergo slight changes, which are of no 

 importance in practice, and this result has been confirmed 

 by various investigators. 



On the other hand, he found that, when the life conditions 

 of the yeast are disturbed by a systematic and more vigorous 

 treatment, it was possible to produce varieties, which remained 

 more or less constant in their properties, and even to produce 

 new species. As a result of these investigations HANSEN 

 obtained useful varieties of some brewery yeasts. 



The biological and physiological characteristics of the 

 species discovered by HANSEN led him also to a method for 

 the practical analysis of brewery yeast, by means of which it is 

 possible to ensure in time against the prevalence of foreign 

 yeasts. It was proved by his experiments on a large scale 

 that the forms which produce yeast-turbidity may be present 

 to the extent of one part in forty-one of the pitching-yeast, 

 and the species (Sacch. Pastorianus I.) which produces a dis- 

 agreeable odour and an objectionable bitter taste to the extent 

 of one part in twenty-two, without exercising any injurious 

 influence, provided the brewing operations are conducted under 



