Obstructions in Draining- Tiles. 



(527 



these deposits Is necessarily very variable. It depends doubtless 

 on the nature of the soil traversed by the waters which produce 

 it. On the other hand, the deposits are almost always mechani- 

 cally compounded with indeterminate, but often considerable, 

 proportions of clay, of fine sand, and of vegetable detritus. 



To give an idea of the differences of composition which exist 

 between one sample and another, the three following analyses are 

 supplied : — 



Sand, iron, and clay insoluble in hydrochloric acid i 17-00 



Alumina 3*67 



Oxide of iron ' 37*67 



Carbonate of lime 6 '33 



Carbonate of magnesia 



Water in combination, substances not proportioned, 



and combustible organic matter, azote not in-} 34 "G 



eluded 



Azote 0-G6 



Sample No. 1 was collected in the environs of Cassel ; it was 

 only dried in the air. Tlie otlier two, before analysis, were dried 

 at a temperature of about 80°. Sample 2 was collected in the 

 environs of Arras, and sample 3 comes from Henonville (Oise). 

 I boiled 100 portions of these samples with potass. 



Sample 2. Sample 3, 



Silica 7-63 5-35 



Alumina traces 2-15 



7-63 7-50 



An analogous deposit, collected at Drayton Manor, and ana- 

 lysed by Mr. Phillips of London, furnished — 



Silica ami ahimina, witli traces of lime .. 49'20 

 'J7-80 



Peroxide oi iron 

 Ortianic matter 



•00 



1 00-00 



It would be difficult, as I said at the beginning, to deduce very 

 useful conclusions from these figures without more detailed re- 

 search. It is not the same with regard to the following facts, of 

 which the j)ra(tical interest will easily be appreciateil. When 

 you collect a recent (lej)osit, and some of the water from whicli 

 it is formed, l)y passing tlic whole through a filter you will 

 obtain a liquid perfectly pure. This liquid, enclosed in Ijottles, 

 entirely filled, and well corked, or placed in an atmosjihere quite 

 deprived of oxygen, preserves its transparency indefinitelv. ['Ex- 

 posed to the action of oxygen, or of atmospheric air, it becomes 



