288 PROFESSOR LETTS ON PHOSPHORUS-BETAINES. 
Respecting compounds of oxygen and of hydrogen with the three ele- 
ments, it may not be superfluous to point out some of the more important and 
interesting points of resemblance and difference which exist between them. 
As regards their compounds with hydrogen, nitrogen forms a single 
hydride ; sulphur, two; phosphorus, three. In all three cases the hydride 
containing the maximum of hydrogen is gaseous, and possesses a powerful and 
characteristic odour and energetic properties. All three of these gaseous 
hydrides are decomposed by the spark, and phosphuretted and sulphuretted 
hydrogen are decomposed by heat. Ammonia, however, is more stable. 
As we might expect from the readiness with which both sulphur and phos- 
phorus are oxidised, their compounds with hydrogen are very inflammable, 
whilst ammonia can only be burnt under special conditions. 
The strongest point of analogy between ammonia and phosphuretted 
hydrogen is, that both are alkaline substances, in which respect they are unique 
amongst the hydrides of elements. But the alkaline properties of phosphuretted 
hydrogen are very weak, as it combines under ordinary atmospheric pressure 
with only two acids, viz., hydriodic and hydrobromic acids, and its compounds 
with these are so unstable that they dissociate at ordinary temperatures, and 
cannot exist in solution. 
As before pointed out, phosphuretted hydrogen, in respect of its alkaline 
properties, is intermediate between the strong base ammonia and the faint acid 
sulphuretted hydrogen. In other respects, phosphuretted hydrogen is more 
allied to sulphuretted hydrogen than it is to ammonia. This is especially » 
noticeable in its action on solutions of the heavy metals, where it acts either 
as a reducing agent (gold, &c.) or precipitates a metallic phosphide (cadmium 
and copper), or precipitates a mixture of the metal and metallic phosphide 
(mercury). 
Both sulphur and phosphorus form only two well-marked compounds with 
oxygen ; whilst nitrogen, in spite of its slight affinity for that element, forms no 
less than five oxides. 
Phosphorus, as we might expect from its powerful affinity for oxygen, 
combines directly with the maximum quantity of that element ; whilst sulphur, 
when burnt, only forms its lower oxide ; and free nitrogen is not capable of direct 
oxidation, except under special conditions. 
The highest oxides of the three elements resemble each other in being 
volatile solids, and in having a strong affinity for water. Nitric anhydride is 
the least stable, and frequently decomposes spontaneously. Sulphuric anhy- 
dride is decomposed at a high teniperature, whilst phosphoric anhydride dis- 
plays a much higher degree of stability. 
If we consider the oxy-acids of the three elements, we see that an undoubted 
analogy exists between sulphuric and phosphoric acids. Both are very powerful 
oaks oe 
