VOYAGE TO ALGERIA. 167 



twenty bottles were filled with water, taken at different 

 places between Oran and Spithead. 



And here let me express my warmest acknowledg- 

 ments to Captain Henderson, the commander of H.M.S. 

 * Urgent,' who aided me in my observations in every 

 possible way. Indeed, my thanks are due to all the 

 officers for their unfailing courtesy and help. The 

 captain placed at my disposal his own coxswain, an 

 intelligent fellow named Thorogood, who skilfully 

 attached a cord to each bottle, weighted it with lead, 

 cast it into the sea, and, after three successive rinsings, 

 filled it under my own eyes. The contact of jugs, 

 buckets, or other vessels was thus avoided ; and even 

 the necessity of pouring out the water, afterwards, 

 through the dirty London air. 



The mode of examination applied to these bottles 

 has been already described. 1 The liquid is illuminated 

 by a powerfully condensed beam, its condition being 

 revealed through the light scattered by its suspended 

 particles. ' Care is taken to defend the eye from the 

 access of all other light, and, thus defended, it becomes 

 an organ of inconceivable delicacy.' Were water of 

 uniform density perfectly free from suspended matter, 

 it would, in my opinion, scatter no light at all. The 

 track of a luminous beam could not be seen in such 

 water. But ' an amount of impurity so infinitesimal as 

 to be scarcely expressible in numbers, and the individual 

 particles of which are so small as wholly to elude the 

 microscope, may, when examined by the method alluded 

 to, produce not only sensible, but striking, effects upon 

 the eye.' 



The results of the examination of nineteen bottles 

 filled at various places between Gibraltar and Spithead 

 are here tabulated : 



* Floating Matter of the Air,' Art. 'Dust and Disease/ 

 12 



