[SDL] Questions on Alpha channel

[Lilith Calbridge]

Right now all I have is the documentation on the SDL API.  I have to
gather some information from putting a lot of it together in my head. 
However, without an overview things aren't always as clear to me so I
turn to this list for a bit of help.

I'm in test phase right now just to try out some techniques, mostly
with what might be called particles but mostly just to familiarize
myself with a vector container.  I'll eventually be trying to make
particles using a variable alpha value but for now it's just a fixed
value.

I'm a bit mystified at what I need to set to what.  I've got my video
surface and a background surface which is currently all white.  I blit
the background to the screen and then place each of the particles
individually on the screen.  When I set the screen, via SDL_SetAlpha
with SDL_SRCALPHA it doesn't make any difference what value I set the
pixel alpha to.  If I set the per surface alpha to something approaching
zero the screen fades in to a light grey but it still seems to have
little effect on the alpha appearance of the particles, which are almost
transparent.

What exactly am I missing?  

TIA,
Lilith