114 ReEporT OF DEPARTMENT OF BACTERIOLOGY OF THE 
forms, but no determination was made of the exact conditions 
under which they were produced. 
Potato discolored. The difficulty with this determination 
lay not so much in a variation in the cultures as in the dif- 
ferent conceptions which various workers had of the meaning 
of the term. As used in the table it refers to the formation 
of the nearly white halo which is seen around the margin of 
the growth with certain species of bacteria. This was not 
observed with this collection of cultures. On the contrary, 
the portion of the potato not covered with the luxuriant 
glistening, creamy growth was ordinarily darkened to a faint 
brownish gray, the extent of this change varying apparently 
‘with the characteristics of the potato itself. 
Potato starch destroyed. This item has been added to the 
classification card recently and the determinations were made. 
at the New York laboratory only. The test was made on 
sterilized potato cylinders in test tubes upon which cultures 
were grown for two weeks. These potato cultures were crushed 
in porcelain mortars and 50 cc. of water added to separate 
the particles so that the effect of the iodine could be easily 
seen. The presence of unchanged starch was determined by 
the gradual addition of a weak solution of iodine in potassium 
iodide. After a sufficient amount has been added to satisfy 
some of the other compounds which are present the starch 
grains are turned to a blue-black. The extent of the reduction 
of the starch by the bacterial growth is estimated on the basis 
of similar tests of uninoculated potato cylinders. In none of 
these cultures was the starch reduction complete, but amyl- 
odextrin was formed. (See page 174.) This change was ex- 
pressed by 2 in the appropriate place in the group number. 
For the details of this method of determining this action 
on the starch, we are indebted to Dr. Erwin F. Smith who 
assisted us by correspondence and by a demonstration in 
connection with the meeting of the Society of American 
Bacteriologists at Baltimore. 
Uschinsky’s Solution. The advantage of this solution lies 
in the definite chemical compounds used in its preparation 
