a catalyst and the mixture is placed in a glass reaction vessel 30 mm in diameter equipped with a stirrer a stream of gaseous ch 3 cl is sent via a sintered glass disk supporting the powder the gas flow rate is kept constant at 3 6 10 u2212 3 m 3 hr after heating the reaction medium and starting the reaction the system is maintained at 300 u00b0 c after 12 hours of reaction the mean flow rate obtained in dimethyldichlorosilane is noted along with the content of this product in all the reaction products for the evaluation of the particle size grade of the powder of sample no 1 two types of measurements were made laser granulometry a simplified test with reference to the test described above by working on the test powder directly without adding catalyst at ambient temperature with no heating and by replacing the ch 3 cl gas by nitrogen the laser granulometry detected 5 5 by weight of fines having a size less than 5 u03bcm in the simplified test after 12 hours of treatment the product remaining in the reaction vessel was retrieved and weighed of the 40 g of the initial product only 37 2 g remained i e a loss of 7 sample no 2 prepared at.