122 BACTERIOLOGICAL DIAGNOSIS. 
The pus in these cases is thick and creamy, and of a 
greenish colour; after it has stood for ‘some time a thin 
layer of a greenish fluid appears upon the surface. 
When an empyema is due to the pneumococcus alone, 
no other organisms being present, the prognosis is dis- 
tinctly better than in cases in which other organisms 
are present, and the patient has been known to recover 
after simple aspiration. 
The streptococcus is also readily demonstrated by a 
simple microscopical examination; it grows readily on 
agar, forming small round colonies which do not tend 
to coalesce and are more opaque in the centre than in 
the periphery. 
The pus is not generally very thick, and has a yellow 
colour. It separates into two layers, the upper trans- 
parent layer being much more abundant than is the case 
with pneumococcic pus. 
This is the commonest and most dangerous form of 
empyema. ‘Thorough drainage is essential. 
Staphylococcic empyemata, according to Netter, are 
very rare; the single case in which he found the staphy- 
lococcus alone was secondary to ulcerative endocarditis. 
He also states that when this organism is found in the 
pus tubercle bacilli are often present as well. 
The prognosis of these cases, therefore, appears to be 
bad. 
The tubercle bacillus is responsible for a compara- 
tively small number of cases; and the results of opera- 
tive interference are not gratifying. The prognosis is 
worse than in any other form of the disease. 
The diagnosis may be made from a careful micro- 
scopical examination, but to this end it must be careful, 
as the bacilli are present in but scanty numbers. 
If wo organisms are found after a thorough micro- 
