For the positive constant number $ a$, let $ D$ be the part surrounded by the curve $ \sqrt{x}\plus{}\sqrt{y}\equal{}\sqrt{a}$ and  the line $ x\plus{}y\equal{}a$.

(1) Draw the outline of $ D$ and find the area.

(2) Find the volume of the solid by rotating $ D$ about the line $ x\plus{}y\equal{}a$ as the rotation axis.
### Part 1: Drawing the Outline and Finding the Area

1. **Transform the Equations:**
   - The curve $\sqrt{x} + \sqrt{y} = \sqrt{a}$ can be rewritten as $x + y + 2\sqrt{xy} = a$.
   - The line $x + y = a$ remains unchanged.

2. **Rotation Transformation:**
   - We employ a rotation of $\pi/4$ radians counterclockwise about the origin: 
     \[
     (u,v) = \left(\frac{x - y}{\sqrt{2}}, \frac{x + y}{\sqrt{2}}\right)
     \]
   - Under this transformation, the line $x + y = a$ becomes $v = \frac{a}{\sqrt{2}}$.

3. **Transform the Curve:**
   - Substitute $x = \frac{u + v}{\sqrt{2}}$ and $y = \frac{v - u}{\sqrt{2}}$ into the curve equation:
     \[
     \sqrt{\frac{u + v}{\sqrt{2}}} + \sqrt{\frac{v - u}{\sqrt{2}}} = \sqrt{a}
     \]
   - Simplify to get:
     \[
     v = \frac{u^2}{a\sqrt{2}} + \frac{a}{2\sqrt{2}}
     \]
   - This describes an arc of a parabola over the interval $-\frac{a}{\sqrt{2}} \le u \le \frac{a}{\sqrt{2}}$.

4. **Calculate the Area:**
   - The area $|D|$ is given by the integral:
     \[
     |D| = 2 \int_{u=0}^{a/\sqrt{2}} \left( \frac{a}{\sqrt{2}} - \left( \frac{u^2}{a\sqrt{2}} + \frac{a}{2\sqrt{2}} \right) \right) du
     \]
   - Simplify the integrand:
     \[
     |D| = 2 \int_{u=0}^{a/\sqrt{2}} \left( \frac{a}{2\sqrt{2}} - \frac{u^2}{a\sqrt{2}} \right) du
     \]
   - Evaluate the integral:
     \[
     |D| = 2 \left[ \frac{a}{2\sqrt{2}} u - \frac{u^3}{3a\sqrt{2}} \right]_{0}^{a/\sqrt{2}}
     \]
     \[
     |D| = 2 \left( \frac{a^2}{4} - \frac{a^2}{6} \right) \frac{1}{\sqrt{2}}
     \]
     \[
     |D| = 2 \left( \frac{a^2}{12} \right) \sqrt{2}
     \]
     \[
     |D| = \frac{a^2}{3}
     \]

### Part 2: Finding the Volume of the Solid

1. **Volume Calculation:**
   - The volume $|V|$ is given by rotating the region $D$ about the line $x + y = a$.
   - Using the disk method, the volume is:
     \[
     |V| = 2 \int_{u=0}^{a/\sqrt{2}} \pi \left( \frac{a}{2\sqrt{2}} - \frac{u^2}{a\sqrt{2}} \right)^2 du
     \]
   - Simplify the integrand:
     \[
     |V| = 2\pi \int_{u=0}^{a/\sqrt{2}} \left( \frac{a^2}{8} - \frac{u^2}{2} + \frac{u^4}{2a^2} \right) du
     \]
   - Evaluate the integral:
     \[
     |V| = 2\pi \left[ \frac{a^2}{8} u - \frac{u^3}{6} + \frac{u^5}{10a^2} \right]_{0}^{a/\sqrt{2}}
     \]
     \[
     |V| = 2\pi \left( \frac{a^3}{8\sqrt{2}} - \frac{a^3}{6\sqrt{2}} + \frac{a^3}{10\sqrt{2}} \right)
     \]
     \[
     |V| = 2\pi \left( \frac{5a^3 - 4a^3 + 3a^3}{40\sqrt{2}} \right)
     \]
     \[
     |V| = 2\pi \left( \frac{4a^3}{40\sqrt{2}} \right)
     \]
     \[
     |V| = \frac{\pi \sqrt{2}}{15} a^3
     \]

The final answer is $\boxed{\frac{\pi \sqrt{2}}{15} a^3}$