The term _respirable_, in its physiological application, has been
differently employed. Some times by the respirability of a gas has been
meant, its power of supporting life for a great length of time, when
repeatedly applied to the blood in the lungs. At other times all gases
have been considered as respirable, which were capable of introduction
into the lungs by voluntary efforts, without any relation to their
vitality.
In the last sense the word respirable is most properly employed. In
this sense it is used in the following sections.
Non-respirable gases are those, which when applied to the external
organs of respiration, stimulate the muscles of the epiglottis in inch
a way as to keep it perfectly close on the glottis; thus preventing
the smallest particle of gas from entering into the bronchia, in spite
of voluntary exertions; such are carbonic acid, and acid gases in
general.[183]
[183] See the curious experiments of Rosier, Journal de Physique, 1786,
vol. 1, pag. 419.
Of respirable gases, or those which are capable of being taken into the
lungs by voluntary efforts.
One only has the power of uniformly supporting life;—atmospheric air.
Other gases, when respired, sooner or later produce death; but in
different modes.
Some, as nitrogene and hydrogene, effect no positive change in the
venous blood. Animals immersed in these gases die of a disease produced
by privation of atmospheric air, analogous to that occasioned by their
submersion in water, or non-respirable gases.
Others, as the different varieties of hydrocarbonate, destroy life
by producing some positive change[184] in the blood, which probably
immediately renders it incapable of supplying the nervous and muscular
fibres with principles essential to sensibility and irritability.
Oxygene, which is capable of being respired for a much greater length
of time than any other gas, except common air, finally destroys life;
first producing changes in the blood, connected with new living
action.[185]
[184] As appears from the experiments of Dr. Beddoes; likewise those of
Mr. Watt.
[185] As appears from the experiments of Lavoisier and Dr. Beddoes; and
as will be seen hereafter.
After experiments, to be detailed hereafter, made upon myself and
others, had proved that nitrous oxide was respirable, and capable
of supporting life for a longer time than any of the gases, except
atmospheric air and oxygene, I was anxious to ascertain the effects of
it upon animals, in cases where its action could be carried to a full
extent; and to compare the changes occasioned by it in their organs,
with those produced by other powers.