I02 BACTERIA. 



As Hansen's work is the most recent and most complete that 

 has yet been carried out, we may give a short account of 

 the species that he describes in his classification as quoted by 

 Zopf. He divides the yeasts into three groups : Saccharomyces 

 cerevisise, ii-5/« most frequently 8-6/t in diameter, which 

 contains a single species only ; Sacch. Pastorianus, 17-2.5/* 

 average diameter 8-7/*, in which he describes three species, 

 all of them oblong or sausage-shaped ; whilst the third group, 

 Sacch. Ellipsoideus, 13-2.5^ in diameter, the most usual size 

 being about 8-7/t, in which most of the cells are oval or 

 rounded. These forms so overlap one another that by 

 mere microscopic examination it is impossible to determine 

 whether we are dealing with absolutely pure cultures or not. 



Hansen's researches were carried on practically, and on a 

 most extensive scale, in connection with a large brewing 

 industry at Old Carlsberg in Copenhagen, and he very 

 naturally turned his attention to the classification of beer 

 yeasts for the purpose of obtaining pure and healthy yeasts 

 for the production of beer. 



Taking up the subject where Pasteur nad left off he, 

 however, again went over some of the old ground, and 

 studied the various yeasts with the greatest care ; cultivated 

 them in all kinds of media, noted their behaviour under 

 different conditions as regards temperature, moisture, presence 

 or absence of oxygen and the like ; and as the result of 

 prolonged and ingenious experiniental work he was able to 

 dispel much of the inaccuracy that had grown up around the 

 subject, and to give detailed accounts of the life histories of 

 the saccharomyces. He found that the older observers had 

 not understood the conditions under which spore formation 

 occurs ; that it was not possible, as Reess had stated, to classify 

 the saccharomyce by a mere microscopic examination, in 

 which the characters relied upon were necessarily merely the 

 form and size of the cells and spores. He also pointed out 

 that even Reess' observations on the conditions \inder which 

 spores were formed were not to be relied upon. That they 

 were not the result of the yeast being attacked by mould 

 fungi or bacteria, as Reess and Van Tieghem had^ maintained, 

 both of whom believed that spores were evidence of disease ' 

 in the yeast-cells, he was soon convinced. Nor could he 

 believe that Wiesner and Brefeld, who made very careful 

 study of yeasts, without, however, enjoying the advantages of 



