16 PLANKTON OF WISCONSIN LAKES 



chemical analyses. In 1917 only one run per week was made. On 

 Lakes Monona and Waubesa observations were not made at regular in- 

 tervals in 1915, but in 1916 they were made approximately every two 

 weeks. 



Chemical Methods 



The various methods employed in the chemical analyses have been 

 described by Schuette 3 so that they need not be considered in detail 

 here. In all instances the samples used for the analyses were dried 

 in an electric oven at a temperature of 60° C. for a period of twelve 

 hours. After cooling in a desiccator the weights of the samples were 

 taken and these constitute the dry weights used in the calculation of 

 the percentages of the different substances determined. The material 

 is more or less hygroscopic and the moisture lost in this drying process 

 varied from a minimum of about 1 per cent to a maximum of almost 

 7 per cent; in most cases, however, the loss was between 2 per 

 cent and 4 per cent. The material was not dried at a higher tem- 

 perature because it was feared that some of the oils might be lost by 

 volatilization at higher temperatures. Usually from half a gram to 

 a gram of material was used for an analysis and a number of experi- 

 ments showed that this amount came to a constant weight under the 

 conditions described above, thus showing that it was thoroughly dry. 



In determining the ash the sample was first carbonized at a low 

 temperature, after which it was placed in an electric furnace heated 

 to a temperature of 500° to 600°. It usually required from 15 minutes 

 to 20 minutes to complete the ashing process at this temperature. That 

 portion of the material that is consumed may be regarded as the organic 

 matter, but a correction is necessary where a considerable amount of 

 magnesium carbonate is present, since this substance gives off its 

 carbon dioxide at a temperature of 250° to 300°. The regular methods 

 were employed in making the quantitative determinations of the vari- 

 ous constituents of the ash. 



All nitrogen determinations were made by the Kjeldahl-Gunning- 

 Arnold method. 



The ether extract was determined in the Soxhlet extraction ap- 

 paratus and the extraction process was continued for a period of 24 

 hours. In addition to the oils and fats, chlorophyl is also extracted 

 by the ether but the quantity of this substance is regarded as too small 

 to affect the results materially. 



In determining the pentosans the material was distilled with hydro- 

 chloric acid, specific gravity 1.06, and the furfurol in the filtered dis- 

 tillate was precipitated with phloroglucinol solution. After standing 



"Trans. Wis. Acad. Sei. Arts, and Let., Vol. XIX, 1918, p. 594. 



