![{\displaystyle \left.{\begin{aligned}&\left({\frac {\operatorname {d} y}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}z}{\operatorname {d} t^{2}}}-{\frac {\operatorname {d} z}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}y}{\operatorname {d} t^{2}}}\right)(X-x)\\\\+&\left({\frac {\operatorname {d} z}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}x}{\operatorname {d} t^{2}}}-{\frac {\operatorname {d} x}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}z}{\operatorname {d} t^{2}}}\right)(Y-y)\\\\+&\left({\frac {\operatorname {d} x}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}y}{\operatorname {d} t^{2}}}-{\frac {\operatorname {d} y}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}x}{\operatorname {d} t^{2}}}\right)(Z-z)\end{aligned}}\right\}=0\,;\quad }](https://wikimedia.org/api/rest_v1/media/math/render/svg/778297f0143e6df569afae55198544e028eefcd6)
(7)
et il faudra d’abord que ce plan soit perpendiculaire, en
au plan tangent (5) au même point ; ce qui donnera, pour première équation du mouvement de la molécule
![{\displaystyle P\left({\frac {\operatorname {d} y}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}z}{\operatorname {d} t^{2}}}-{\frac {\operatorname {d} z}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}y}{\operatorname {d} t^{2}}}\right)+Q\left({\frac {\operatorname {d} z}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}x}{\operatorname {d} t^{2}}}-{\frac {\operatorname {d} x}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}z}{\operatorname {d} t^{2}}}\right)}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0e9a827d0ae384d0759cb2e7c7609cbf0ec46fad)
![{\displaystyle +R\left({\frac {\operatorname {d} x}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}y}{\operatorname {d} t^{2}}}-{\frac {\operatorname {d} y}{\operatorname {d} t}}{\frac {\operatorname {d} ^{2}x}{\operatorname {d} t^{2}}}\right)=0,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5c65e7762a8c31c98b3fdd83128b4e4654d1635d)
ou bien
![{\displaystyle \left(Q{\frac {\operatorname {d} ^{2}z}{\operatorname {d} t^{2}}}-R{\frac {\operatorname {d} ^{2}y}{\operatorname {d} t^{2}}}\right){\frac {\operatorname {d} x}{\operatorname {d} t}}+\left(R{\frac {\operatorname {d} ^{2}x}{\operatorname {d} t^{2}}}-P{\frac {\operatorname {d} ^{2}z}{\operatorname {d} t^{2}}}\right){\frac {\operatorname {d} y}{\operatorname {d} t}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1bf9d7b9d340bd48988517284c0ba425c0100323)
![{\displaystyle +\left(P{\frac {\operatorname {d} ^{2}y}{\operatorname {d} t^{2}}}-Q{\frac {\operatorname {d} ^{2}x}{\operatorname {d} t^{2}}}\right){\frac {\operatorname {d} z}{\operatorname {d} t}}=0.\quad }](https://wikimedia.org/api/rest_v1/media/math/render/svg/4c07ef42eb6d28301e955103ecf66f1e0b4cc2b6)
(8)
Les vîtesses de la molécule, parallèlement aux axes des
, des
et des
, étant respectivement
![{\displaystyle {\frac {\operatorname {d} x}{\operatorname {d} t}},\quad {\frac {\operatorname {d} y}{\operatorname {d} t}},\quad {\frac {\operatorname {d} z}{\operatorname {d} t}}\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d85408db055c45d9869314bda20ce5f3fff24a97)
les équations de la tangente à la trajectoire, au point
seront
![{\displaystyle {\frac {X-x}{\frac {\operatorname {d} x}{\operatorname {d} t}}}={\frac {Y-y}{\frac {\operatorname {d} y}{\operatorname {d} t}}}={\frac {Z-z}{\frac {\operatorname {d} z}{\operatorname {d} t}}}\,;\qquad }](https://wikimedia.org/api/rest_v1/media/math/render/svg/873afe8348d2e65ab4598b1a3bfab89cb8ba36ad)
(9)
de sorte que, si l’on représente par
l’angle que fait cette tan-