930 



SCIENCE 



[N. S. Vol. XXVIII. No. 730 



The Museum News of the Brooklyn Insti- 

 tute notices the installation of an exhibit 

 illustrating Mr. Abbott H. Thayer's prin- 

 ciples of obliterative shading, of a group of 

 timber wolves and of a large painting showing 

 the appearance of a coral reef. The main 

 article in the Children's Museum section de- 

 scribes coffee culture in Guatemala. 



SPECIAL ARTICLES 



THE DETERMINATION OF THE CLAY CONTENT 

 OP SOILS"^ 



In the method of mechanical analysis of 

 soils originally devised and used in the 

 Bureau of Soils of the United States Depart- 

 ment of Agriculture," the amount of clay was 

 determined — after the coarser particles had 

 been separated by centrifuging — by evapora- 

 ting the clay-water to small bulk in enameled- 

 ware sauce pans, transferring it to platinum, 

 carrying completely to dryness, and weighing 

 the residue. This method was found unde- 

 sirable on account of the excessive time re- 

 quired for the evaporation, because many dust 

 particles and fragments cracked from the lin- 

 ing of the sauce pans got into the clay and 

 were weighed with it, and because the transfer 

 from sauce pans to platinum required much 

 time and trouble and offered possibilities of 

 loss. To avoid these difficulties, and espe- 

 cially to increase the rapidity of the work, it 

 was therefore decided to abandon the final 

 transfer to platinum, and to complete the 

 evaporation in the enameled-ware dishes and 

 weigh the clay in them without transfer. 

 Tor convenience in weighing and to decrease 

 the likelihood of entry of dust, the enameled- 

 ware sauce pans were discarded and there 

 were adopted instead much smaller enameled- 

 ware cups, having a capacity of about 300 c.c. 

 and weighing about 180 grams. This method 

 has the disadvantage of requiring a much 

 heavier balance, but the determinations seem 

 none the less accurate, and a great saving of 

 time has been effected. 



It is now thought that the method may be 



^Published by permission of the Secretary of 

 Agriculture. 



''See Bulletin No. 24, Bureau of Soils, U. S. 

 Department of Agriculture (1904). 



still further shortened in the case of routine 

 analyses by abandoning altogether the direct 

 determination of clay and obtaining its per- 

 centage by difference. The error thus intro- 

 duced wiU probably be far less than that in- 

 volved in the centrifugal (or other) separa- 

 tion of the clay from the silt, and both errors 

 are almost certainly within the limits of varia- 

 tion which may be expected between different 

 samples of the same soil. In one hundred 

 consecutive analyses taken at random from the 

 recent files of the Bureau of Soils the varia- 

 tions between the percentage of clay as 

 directly determined and that found by differ- 

 ence ranged between and 2.28 per cent. In 

 two cases the variation was over 2.00 per 

 cent.; in three cases, between 2.00 per cent, 

 and 1.50 per cent. ; and in six cases between 

 1.50 per cent, and 1.00 per cent. In all other 

 cases the variations were 1.00 per cent, or less. 



It is indeed not improbable that in many 

 eases the determination by difference is the 

 more accurate, as all errors due to access of 

 dust or to loss of clay water are thus avoided. 

 Errors due to a not impossible change in the 

 state of hydration of the clay are also elimi- 

 nated. It is believed that in the vast ma- 

 jority of cases the difference method in 

 clay determination will be accurate within 

 1.00 per cent., and no greater accuracy is 

 necessary or even desirable for purposes of 

 soil classification. The saving in time is at 

 least 30 per cent., and is believed to be well 

 worth while in routine laboratories making a 

 large number of analyses. The direct de- 

 termination must, of course, be retained for 

 cases of especial importance, or where a check 

 is necessary and is probably also advisable for 

 students' use, and for use in small labora- 

 tories. 



C. 0. Fletcher 



BuEEATj OP Soils, 

 U. S. Department of Ageicultuee 



NOTES ON THE ATROPHY OP THE EYE OP RAJA 

 ERINACEA 



In the skate, Baja erinacea, a clearly defined 

 case of complete atrophy of the left eyeball 

 was observed in our course this fall. 



The brownish integument continued without 



