256 Dr Davy's Miscellaneous Observations, 



of impalpable powder, merely adhering to the siliceous sand, 

 which was in the form of grains of transparent colourless 

 quartz, most of them rounded, having been water- worn : such 

 was their appearance under the microscope. 



I did not think it necessary to make the experiment on the 

 change of colour out of the influence of atmospheric air, tak- 

 ing it for granted that oxygen was in no wise concerned in 

 producing the change ; and that this is another instance (if 

 it has not been noticed before) of change of colouring de- 

 pending on change of temperature, and independent of any 

 change in the chemical composition of the substance exhibit- 

 ing it. 



Trials made on soils of excellent quality, — dark mould con- 

 taining very little vegetable matter, — have shewn the same 

 change of colour from change of temperature, as the coloured 

 sand just mentioned. 



These instances seem deserving of being kept in mind in 

 conducting the examination of a soil containing iron, — a com- 

 mon ingredient of soils, and in the state of peroxide. Mere 

 blackening of such a soil should not be considered a satisfac- 

 tory proof of the presence of vegetable matter. More deci- 

 sive proof should be required, afforded by the smell on the 

 first application of heat, and by comparison of colour before 

 and after a sufficient application to destroy any vegetable 

 matter which may be present. 



2. When iron is precipitated by ferrocyanide of potassium 

 in a solution containing alumina, and the latter, without se- 

 paration of the precipitate by filtration, is thrown down by 

 ammonia, the colour of the first from blue becomes brown, 

 as if originally precipitated by ammonia. Now, after having 

 been well washed, after having been collected in a filter, if 

 sulphuric or any other mineral acid be added, the mixed pre- 

 cipitate will acquire a blue colour; and it is surprising how 

 long after repeated washings it retains this property, gradual- 

 ly, however, diminishing in intensity of colour. 



Other instances might be given of the power of precipitates 

 to retain a portion of another matter, existing in the solution 

 from which they have been thrown down. I shall mention 

 one only in particular. It occurred in examining cane-ashes 



